Boolean algebra, developed in 1854 by George Boole in his book An Investigation of the Laws of Thought, is a variant of ordinary elementary algebra. Boolean algebra differs from ordinary algebra in three ways: in the values that variables may assume, which are of a logical instead of a numeric character, prototypically 0 and 1; in the operations applicable to those values; and in the properties of those operations, that is, the laws they obey. Applications include mathematical logic, digital logic, computer programming, set theory, and statistics.
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Whereas elementary algebra deals mainly with real numbers, Boolean algebra deals with the values 0 and 1. These can be thought of as two integers, or as the truth values false and true respectively. In either case they are called bits or binary digits, in contrast to the decimal digits 0 through 9.
Boolean algebra also deals with other values on which Boolean operations can be defined, such as sets or sequences of bits. However Boolean algebra is unlike many other systems of algebra in that it obeys exactly the same laws (equational properties), neither more nor fewer, no matter which of these other values are employed. Much of the subject can therefore be introduced without reference to any values besides 0 and 1. We therefore postpone to the section on Boolean algebras the treatment of these other values and focus on these two prototypical Boolean values.
Some operations of ordinary algebra, in particular multiplication xy, addition x+y, and negation x, have their counterparts in Boolean algebra, respectively the Boolean operations AND, OR, and NOT, also called conjunction x∧y, disjunction x∨y, and negation or complement ¬x or sometimes !x. Some authors use instead the same arithmetic operations as ordinary algebra reinterpreted for Boolean algebra, treating xy as synonymous with x∧y and x+y with x∨y.
Conjunction x∧y behaves on 0 and 1 exactly as multiplication does for ordinary algebra: if either x or y is 0 then x∧y is 0, but if both are 1 then x∧y is 1.
Disjunction x∨y works almost like addition, with 0∨0 = 0 and 1∨0 = 0∨1 = 1. However there is a difference: 1∨1 is not 2 but 1.
Complement resembles ordinary negation in that it exchanges values. But whereas in ordinary algebra negation interchanges 1 and 1, 2 and 2, etc. while leaving 0 fixed, in Boolean algebra complement interchanges 0 and 1. One can think of ordinary negation as reflecting about 0, and Boolean complement as reflecting about the midpoint of 0 and 1. Complement can be defined arithmetically as ¬x = 1−x.
In summary the three basic Boolean operations can be defined arithmetically as follows.
x∧y  =  xy  
x∨y  =  x + y  xy  
¬x  =  1  x 
Alternatively the values of x∧y, x∨y, and ¬x can be expressed without reference to arithmetic operations by tabulating their values with truth tables as follows.


For the two binary operations ∧ and ∨ the truth tables list all four possible combinations of values for x and y, one per line. For each combination the truth tables tabulate the values of x∧y and x∨y. The truth values of ¬x are tabulated similarly except that only two lines are needed because there is only one variable.
Yet another way of specifying these operations is with equations explicitly giving their values.
0∧0  =  0  0∨0  =  0 
0∧1  =  0  0∨1  =  1 
1∧0  =  0  1∨0  =  1 
1∧1  =  1  1∨1  =  1 
¬0  =  1  ¬1  =  0 
We have so far seen three Boolean operations. We referred to these as basic, meaning that they can be taken as a basis for other Boolean operations that can be built up from them by composition, the manner in which operations are combined or compounded. Here are some examples of operations composed from the basic operations.
x → y  =  ¬(x ∧ ¬y) 
x ⊕ y  =  (x ∨ y) ∧ ¬(x ∧ y) 
x ≡ y  =  ¬(x ⊕ y) 
These definitions give rise to the following truth tables giving the values of these operations for all four possible inputs.
x  y  x → y  x ⊕ y  x ≡ y  

0  0  1  0  1  
1  0  0  1  0  
0  1  1  1  0  
1  1  1  0  1 
The first operation, x → y, is called material implication. If x is true then the value of x → y is taken to be that of y. But if x is false then we ignore the value of y; however we must return some truth value and there are only two choices, so we choose the value that entails less, namely true. (Relevance logic addresses this by viewing an implication with a false premise as something other than either true or false.)
The second operation, x ⊕ y, is called exclusive or to distinguish it from disjunction as the inclusive kind. It excludes the possibility of both x and y. Defined in terms of arithmetic it is addition mod 2 where 1 + 1 = 0.
The third operation, the complement of exclusive or, is equivalence or Boolean equality: x ≡ y is true just when x and y have the same value. Hence x ⊕ y as its complement can be understood as x ≠ y, being true just when x and y are different.
A law of Boolean algebra is an equation such as x∨(y∨z) = (x∨y)∨z between two Boolean terms, where a Boolean term is defined as an expression built up from variables and the constants 0 and 1 using the operations ∧, ∨, and ¬. The concept can be extended to terms involving other Boolean operations such as ⊕, →, and ≡, but such extensions are unnecessary for the purposes to which the laws are put. Such purposes include the definition of a Boolean algebra as any model of the Boolean laws, and as a means for deriving new laws from old as in the derivation of x∨(y∧z) = x∨(z∧y) from y∧z = z∧y as treated in the section on axiomatization.
Boolean algebra satisfies many of the same laws as ordinary algebra when we match up ∨ with addition and ∧ with multiplication. In particular the following laws are common to both kinds of algebra.
(Associativity of ∨)  x∨(y∨z)  =  (x∨y)∨z 
(Associativity of ∧)  x∧(y∧z)  =  (x∧y)∧z 
(Commutativity of ∨)  x∨y  =  y∨x 
(Commutativity of ∧)  x∧y  =  y∧x 
(Distributivity of ∧ over ∨)  x∧(y∨z)  =  (x∧y)∨(x∧z) 
(Identity for ∨)  x∨0  =  x 
(Identity for ∧)  x∧1  =  x 
(Annihilator for ∧)  x∧0  =  0 
Boolean algebra however obeys some additional laws, in particular the following.
(Idempotence of ∨)  x∨x  =  x 
(Idempotence of ∧)  x∧x  =  x 
(Absorption 1)  x∧(x∨y)  =  x 
(Absorption 2)  x∨(x∧y)  =  x 
(Distributivity of ∨ over ∧)  x∨(y∧z)  =  (x∨y)∧(x∨z) 
(Annihilator for ∨)  x∨1  =  1 
A consequence of the first of these laws is 1∨1 = 1, which is false in ordinary algebra, where 1+1 = 2. Taking x = 2 in the second law shows that it is not an ordinary algebra law either, since 2×2 = 4. The remaining four laws can be falsified in ordinary algebra by taking all variables to be 1, for example in Absorption Law 1 the left hand side is 1(1+1) = 2 while the right hand side is 1, and so on.
All of the laws treated so far have been for conjunction and disjunction. These operations have the property that changing either argument either leaves the output unchanged or the output changes in the same way as the input. Equivalently, changing any variable from 0 to 1 never results in the output changing from 1 to 0. Operations with this property are said to be monotone. Thus the axioms so far have all been for monotonic Boolean logic. Nonmonotonicity enters via complement ¬ as follows.
The complement operation is defined by the following two laws.
(Complementation 1)  x∧¬x  =  0 
(Complementation 2)  x∨¬x  =  1. 
All properties of negation including the laws below follow from the above two laws alone.
In both ordinary and Boolean algebra, negation works by exchanging pairs of elements, whence in both algebras it satisfies the double negation law
(Double negation)  ¬¬x = x. 
But whereas ordinary algebra satisfies the two laws
(x)(y)  =  xy  
(x)+(y)  =  (x+y), 
Boolean algebra satisfies De Morgan's laws,
(De Morgan 1)  (¬x)∧(¬y)  =  ¬(x∨y) 
(De Morgan 2)  (¬x)∨(¬y)  =  ¬(x∧y). 
The laws Complementation 1 and 2, together with the monotone laws form a complete set of laws, or axiomatization, in two senses, semantic and syntactic, treated in the sections below on respectively Boolean algebras and their axiomatization. In the semantic sense, every set with operations ∧, ∨, and ¬ satisfying these laws, called a model of the laws, is a Boolean algebra. A set of laws would be incomplete in this sense when it has models that are not Boolean algebras. In the syntactic sense, every equation between Boolean terms that holds for all ways of assigning 0 or 1 to the variables in the terms, called a tautology, is a logical consequence of the laws. If a tautology does not follow from a set of laws, that set is incomplete in this second sense. The two senses coincide with regard to which sets of laws they define as being complete. This correspondence between syntax and semantics is not specific to Boolean algebra but holds of any equational theory.
There is nothing magic about the choice of symbols for the values of Boolean algebra. We could rename 0 and 1 to say α and β, and as long as we did so consistently throughout it would still be Boolean algebra, albeit with some obvious cosmetic differences.
But suppose we rename 0 and 1 to 1 and 0 respectively. Then it would still be Boolean algebra, and moreover operating on the same values. However it would not be identical to our original Boolean algebra because now we find ∨ behaving the way ∧ used to do and vice versa. So there are still some cosmetic differences to show that we've been fiddling with the notation, despite the fact that we're still using 0's and 1's.
But if in addition to interchanging the names of the values we also interchange the names of the two binary operations, now there is no trace of what we have done. The end product is completely indistinguishable from what we started with. We might notice that the columns for x∧y and x∨y in the truth tables had changed places, but that switch is immaterial.
When values and operations can be paired up in a way that leaves everything important unchanged when all pairs are switched simultaneously, we call the members of each pair dual to each other. Thus 0 and 1 are dual, and ∧ and ∨ are dual. The Duality Principle, also called De Morgan duality, asserts that Boolean algebra is unchanged when all dual pairs are interchanged.
One change we did not need to make as part of this interchange was to complement. We say that complement is a selfdual operation. The identity or donothing operation x (copy the input to the output) is also selfdual. A more complicated example of a selfdual operation is (x∧y) ∨ (y∧z) ∨ (z∧x). It can be shown that selfdual operations must take an odd number of arguments; thus there can be no selfdual binary operation.
A Venn diagram is a representation of a Boolean operation using shaded overlapping regions. There is one region for each variable, all circular in the examples here. The interior and exterior of region x corresponds respectively to the values 1 (true) and 0 (false) for variable x. The shading indicates the value of the operation for each combination of regions, with dark denoting 1 and light 0 (some authors use the opposite convention).
The three Venn diagrams in the figure below represent respectively conjunction x∧y, disjunction x∨y, and complement ¬x.
For conjunction, the region inside both circles is shaded to indicate that x∧y is 1 when both variables are 1. The other regions are left unshaded to indicate that x∧y is 0 for the other three combinations.
The second diagram represents disjunction x∨y by shading those regions that lie inside either or both circles. The third diagram represents complement ¬x by shading the region not inside the circle.
While we have not shown the Venn diagrams for the constants 0 and 1, they are trivial, being respectively a white box and a dark box, neither one containing a circle. However we could put a circle for x in those boxes, in which case each would denote a function of one argument, x, which returns the same value independently of x, called a constant function. As far as their outputs are concerned, constants and constant functions are indistinguishable; the difference is that a constant takes no arguments, called a zeroary or nullary operation, while a constant function takes one argument, which it ignores, and is a unary operation.
Venn diagrams are helpful in visualizing laws. The commutativity laws for ∧ and ∨ can be seen from the symmetry of the diagrams: a binary operation that was not commutative would not have a symmetric diagram.
Idempotence of ∧ and ∨ can be visualized by sliding the two circles together and noting that the shaded area then becomes the whole circle, for both ∧ and ∨.
To see the first absorption law, x∧(x∨y) = x, start with the diagram in the middle for x∨y and note that the portion of the shaded area in common with the x circle is the whole of the x circle. For the second absorption law, x∨(x∧y) = x, start with the left diagram for x∧y and note that shading the whole of the x circle results in just the x circle being shaded, since the previous shading was inside the x circle.
The double negation law can be seen by complementing the shading in the third diagram for ¬x, which shades the x circle.
To visualize the first De Morgan's law, (¬x)∧(¬y) = ¬(x∨y), start with the middle diagram for x∨y and complement its shading so that only the region outside both circles is shaded, which is what the right hand side of the law describes. The result is the same as if we shaded that region which is both outside the x circle and outside the y circle, i.e. the conjunction of their exteriors, which is what the left hand side of the law describes.
The second De Morgan's law, (¬x)∨(¬y) = ¬(x∧y), works the same way with the two diagrams interchanged.
The first complement law, x∧¬x = 0, says that the interior and exterior of the x circle have no overlap. The second complement law, x∨¬x = 1, says that everything is either inside or outside the x circle.
Digital logic is the application of the Boolean algebra of 0 and 1 to electronic hardware consisting of logic gates connected to form a circuit diagram. Each gate implements a Boolean operation, and is depicted schematically by a shape indicating the operation. The shapes associated with the gates for conjunction (ANDgates), disjunction (ORgates), and complement (inverters) are as follows.
The lines on the left of each gate represent input wires or ports. The value of the input is represented by a voltage on the lead. For socalled "activehigh" logic 0 is represented by a voltage close to zero or "ground" while 1 is represented by a voltage close to the supply voltage; activelow reverses this. The line on the right of each gate represents the output port, which normally follows the same voltage conventions as the input ports.
Complement is implemented with an inverter gate. The triangle denotes the operation that simply copies the input to the output; the small circle on the output denotes the actual inversion complementing the input. The convention of putting such a circle on any port means that the signal passing through this port is complemented on the way through, whether it is an input or output port.
There being eight ways of labeling the three ports of an ANDgate or ORgate with inverters, this convention gives a wide range of possible Boolean operations realized as such gates so decorated. Not all combinations are distinct however: any labeling of ANDgate ports with inverters realizes the same Boolean operation as the opposite labeling of ORgate ports (a given port of the ANDgate is labeled with an inverter if and only if the corresponding port of the ORgate is not so labeled). This follows from De Morgan's laws.
If we complement all ports on every gate, and interchange ANDgates and ORgates, as in Figure 4 below, we end up with the same operations as we started with, illustrating both De Morgan's laws and the Duality Principle. Note that we did not need to change the triangle part of the inverter, illustrating selfduality for complement.
Because of the pairwise identification of gates via the Duality Principle, even though 16 schematic symbols can be manufactured from the two basic binary gates AND and OR by furnishing their ports with inverters (circles), they only represent eight Boolean operations, namely those operations with an odd number of ones in their truth table. Altogether there are 16 binary Boolean operations, the other eight being those with an even number of ones in their truth table, namely the following. The constant 0, viewed as a binary operation that ignores both its inputs, has no ones, the six operations x, y, ¬x, ¬y (as binary operations that ignore one input), x⊕y, and x≡y have two ones, and the constant 1 has four ones.
The term "algebra" denotes both a subject and an object. Whereas the foregoing has addressed the subject of Boolean algebra, this section deals with mathematical objects called Boolean algebras. We begin with a special case of the notion definable without reference to the laws of Boolean algebra, namely concrete Boolean algebras, and then pass to the general case based on those laws, namely abstract Boolean algebras.
A concrete Boolean algebra, also called a field of sets, is any nonempty set of subsets of a given set X closed under the set operations of union, intersection, and complement relative to X. (As an aside, historically X itself was required to be nonempty as well to exclude the degenerate or oneelement Boolean algebra, which is the one exception to the rule that all Boolean algebras satisfy the same equations since the degenerate algebra satisfies every equation. However this exclusion conflicts with the preferred purely equational definition of "Boolean algebra," there being no way to rule out the oneelement algebra using only equations0 ≠ 1 does not count, being a negated equation. Hence modern authors allow the degenerate Boolean algebra and let X be empty.)
Example 1. The power set 2^{X} of X, consisting of all subsets of X. Here X may be any set: empty, finite, infinite, or even uncountable.
Example 2. The empty set and X. This twoelement algebra shows that a concrete Boolean algebra can be finite even when it consists of subsets of an infinite set. It can be seen that every field of subsets of X must contain the empty set and X. Hence no smaller example is possible, other than the degenerate algebra obtained by taking X to be empty so as to make the empty set and X coincide.
Example 3. The set of finite and cofinite sets of integers, where a cofinite set is one omitting only finitely many integers. This is clearly closed under complement, and is closed under union because the union of a cofinite set with any set is cofinite, while the union of two finite sets is finite. Intersection behaves like union with "finite" and "cofinite" interchanged.
Example 4. For a less trivial example of the point made by Example 2, consider a Venn diagram formed by n closed curves partitioning the diagram into 2^{n} regions, and let X be the (infinite) set of all points in the plane not on any curve but somewhere within the diagram. The interior of each region is thus an infinite subset of X, and every point in X is in exactly one region. Then the set of all 2^{2n} possible unions of regions (including the empty set obtained as the union of the empty set of regions and X obtained as the union of all 2^{n} regions) is closed under union, intersection, and complement relative to X and therefore forms a concrete Boolean algebra. Again we have finitely many subsets of an infinite set forming a concrete Boolean algebra, with Example 2 arising as the case n = 0 of no curves.
A subset Y of X can be identified with an indexed family of bits with index set X, with the bit indexed by x ∈ X being 1 or 0 according to whether or not x ∈ Y. (This is the socalled characteristic function notion of a subset.) For example a 32bit computer word consists of 32 bits indexed by the set {0,1,2,…,31}, with 0 and 31 indexing the low and high order bits respectively. For a smaller example, if X = {a,b,c} where a, b, c are viewed as bit positions in that order, the eight subsets {}, {c}, {b}, {b,c}, {a}, {a,c}, {a,b}, and {a,b,c} of X can be identified with the respective bit vectors 000, 001, 010, 011, 100, 101, 110, and 111. Bit vectors indexed by the set of natural numbers are infinite sequences of bits, while those indexed by the reals in the unit interval [0,1] are packed too densely to be able to write conventionally but nonetheless form welldefined indexed families.
From this bit vector viewpoint, a concrete Boolean algebra can be defined equivalently as a nonempty set of bit vectors all of the same length (more generally, indexed by the same set) and closed under the bit vector operations of bitwise ∧, ∨, and ¬, as in 1010∧0110 = 0010, 1010∨0110 = 1110, and ¬1010 = 0101, the bit vector realizations of intersection, union, and complement respectively.
The set {0,1} and its Boolean operations as treated above can be understood as the special case of bit vectors of length one, which by the identification of bit vectors with subsets can also be understood as the two subsets of a oneelement set. We call this the protypical Boolean algebra, justified by the following observation.
This observation is easily proved as follows. Certainly any law satisfied by all concrete Boolean algebras is satisfied by the prototypical one since it is concrete. Conversely any law that fails for some concrete Boolean algebra must have failed at a particular bit position, in which case that position by itself furnishes a onebit counterexample to that law.
The final goal of the next section can be understood as eliminating "concrete" from the above observation. We shall however reach that goal via the surprisingly stronger observation that, up to isomorphism, all Boolean algebras are concrete.
The Boolean algebras we have seen so far have all been concrete, consisting of bit vectors or equivalently of subsets of some set. Such a Boolean algebra consists of a set and operations on that set which can be shown to satisfy the laws of Boolean algebra.
Instead of showing that the Boolean laws are satisfied, we can instead postulate a set X, two binary operations on X, and one unary operation, and require that those operations satisfy the laws of Boolean algebra. The elements of X need not be bit vectors or subsets but can be anything at all. This leads to the more general abstract definition.
For the purposes of this definition it is irrelevant how the operations came to satisfy the laws, whether by fiat or proof. All concrete Boolean algebras satisfy the laws (by proof rather than fiat), whence every concrete Boolean algebra is a Boolean algebra according to our definitions. This axiomatic definition of a Boolean algebra as a set and certain operations satisfying certain laws or axioms is entirely analogous to the abstract definitions of group, ring, field etc. characteristic of modern or abstract algebra.
Although every concrete Boolean algebra is a Boolean algebra, not every Boolean algebra need be concrete. Let n be a squarefree positive integer, one not divisible by the square of an integer, for example 30 but not 12. The operations of greatest common divisor, least common multiple, and division into n (that is, ¬x = n/x), can be shown to satisfy all the Boolean laws when their arguments range over the positive divisors of n. Hence those divisors form a Boolean algebra. These divisors are not subsets of a set, making the divisors of n a Boolean algebra that is not concrete according to our definitions.
However if we represent each divisor of n by the set of its prime factors, we find that this nonconcrete Boolean algebra is isomorphic to the concrete Boolean algebra consisting of all sets of prime factors of n, with union corresponding to least common multiple, intersection to greatest common divisor, and complement to division into n. So this example while not technically concrete is at least morally concrete via this representation, called an isomorphism. This example is an instance of the following notion.
The obvious next question is answered positively as follows.
That is, up to isomorphism, abstract and concrete Boolean algebras are the same thing. This quite nontrivial result is treated in more detail in the article Boolean algebra (structure), see also Boolean algebras canonically defined. This strong relationship implies a weaker result strengthening the observation in the previous subsection to the following easy consequence of representability.
It is weaker in the sense that it does not of itself imply representability. Boolean algebras are special here, for example a relation algebra is a Boolean algebra with additional structure but it is not the case that every relation algebra is representable in the sense appropriate to relation algebras.
The above definition of an abstract Boolean algebra as a set and operations satisfying "the" Boolean laws raises the question, what are those laws? A simpleminded answer is "all Boolean laws," which can be defined as all equations that hold for the Boolean algebra of 0 and 1. Since there are infinitely many such laws this is not a terribly satisfactory answer in practice, leading to the next question: does it suffice to require only finitely many laws to hold?
In the case of Boolean algebras the answer is yes. In particular the finitely many equations we have listed above suffice. We say that Boolean algebra is finitely axiomatizable or finitely based.
Can this list be made shorter yet? Again the answer is yes. To begin with, some of the above laws are implied by some of the others. A sufficient subset of the above laws consists of the pairs of associativity, commutativity, and absorption laws, distributivity of ∧ over ∨ (or the other distributivity law—one suffices), and the two complement laws. In fact this is the traditional axiomatization of Boolean algebra as a complemented distributive lattice.
By introducing additional laws not listed above it becomes possible to shorten the list yet further, see Boolean algebra (logic). In 1933 Edward Huntington showed that if the basic operations are taken to be x∨y and ¬x, with x∧y considered a derived operation (e.g. via De Morgan's law in the form x∧y = ¬(¬x∨¬y)), then the equation ¬(¬x∨¬y)∨¬(¬x∨y) = x along with the two equations expressing associativity and commutativity of ∨ completely axiomatized Boolean algebra. When the only basic operation is the binary NAND operation ¬(x∧y), Stephen Wolfram has proposed in his book A New Kind of Science the single axiom (((xy)z)(x((xz)x))) = z as a oneequation axiomatization of Boolean algebra, where for convenience here xy denotes the NAND rather than the AND of x and y.
Propositional logic is a logical system that is intimately connected to Boolean algebra. Many syntactic concepts of Boolean algebra carry over to propositional logic with only minor changes in notation and terminology, while the semantics of propositional logic are defined via Boolean algebras in a way that the tautologies (theorems) of propositional logic correspond to equational theorems of Boolean algebra.
Syntactically, every Boolean term corresponds to a propositional formula of propositional logic. In this translation between Boolean algebra and propositional logic, Boolean variables x,y… become propositional variables (or atoms) P,Q,…, Boolean terms such as x∨y become propositional formulas P∨Q, 0 becomes false or ⊥, and 1 becomes true or T. It is convenient when referring to generic propositions to use Greek letters Φ, Ψ,… as metavariables (variables outside the language of propositional calculus, used when talking about propositional calculus) to denote propositions.
The semantics of propositional logic rely on truth assignments. The essential idea of a truth assignment is that the propositional variables are mapped to elements of a fixed Boolean algebra, and then the truth value of a propositional formula using these letters is the element of the Boolean algebra that is obtained by computing the value of the Boolean term corresponding to the formula. In classical semantics, only the twoelement Boolean algebra is used, while in Boolean valued semantics arbitrary Boolean algebras are considered. A tautology is a propositional formula that is assigned truth value 1 by every truth assignment of its propositional variables to an arbitrary Boolean algebra (or, equivalently, every truth assignment to the two element Boolean algebra).
These semantics permit a translation between tautologies of propositional logic and equational theorems of Boolean algebra. Every tautology Φ of propositional logic can be expressed as the Boolean equation Φ = 1, which will be a theorem of Boolean algebra. Conversely every theorem Φ = Ψ of Boolean algebra corresponds to the tautologies (Φ∨¬Ψ) ∧ (¬Φ∨Ψ) and (Φ∧Ψ) ∨ (¬Φ∧¬Ψ). If → is in the language these last tautologies can also be written as (Φ→Ψ) ∧ (Ψ→Φ), or as two separate theorems Φ→Ψ and Ψ→Φ; if ≡ is available then the single tautology Φ ≡ Ψ can be used.
One motivating application of propositional calculus is the analysis of propositions and deductive arguments in natural language. Whereas the proposition "if x = 3 then x+1 = 4" depends on the meanings of such symbols as + and 1, the proposition "if x = 3 then x = 3" does not; it is true merely by virtue of its structure, and remains true whether "x = 3" is replaced by "x = 4" or "the moon is made of green cheese." The generic or abstract form of this tautology is "if P then P", or in the language of Boolean algebra, "P → P".
Replacing P by x = 3 or any other proposition is called instantiation of P by that proposition. The result of instantiating P in an abstract proposition is called an instance of the proposition. Thus "x = 3 → x = 3" is a tautology by virtue of being an instance of the abstract tautology "P → P". All occurrences of the instantiated variable must be instantiated with the same proposition, to avoid such nonsense as P → x = 3 or x = 3 → x = 4.
Propositional calculus restricts attention to abstract propositions, those built up from propositional variables using Boolean operations. Instantiation is still possible within propositional calculus, but only by instantiating propositional variables by abstract propositions, such as instantiating Q by Q→P in P→(Q→P) to yield the instance P→((Q→P)→P).
(The availability of instantiation as part of the machinery of propositional calculus avoids the need for metavariables within the language of propositional calculus, since ordinary propositional variables can be considered within the language to denote arbitrary propositions. The metavariables themselves are outside the reach of instantiation, not being part of the language of propositional calculus but rather part of the same language for talking about it that this sentence is written in, where we need to be able to distinguish propositional variables and their instantiations as being distinct syntactic entities.)
An axiomatization of propositional calculus is a set of tautologies called axioms and one or more inference rules for producing new tautologies from old. A proof in an axiom system A is a finite nonempty sequence of propositions each of which is either an instance of an axiom of A or follows by some rule of A from propositions appearing earlier in the proof (thereby disallowing circular reasoning). The last proposition is the theorem proved by the proof. Every nonempty initial segment of a proof is itself a proof, whence every proposition in a proof is itself a theorem. An axiomatization is sound when every theorem is a tautology, and complete when every tautology is a theorem.
There are many ways of axiomatizing propositional calculus. One style, called a Hilbertstyle deduction system, has very few inference rules, relying primarily on axioms. The commonest example has as its only rule of inference Modus Ponens, from P and P→Q infer Q. A typical axiomatization of propositional calculus using just the connectives → and ¬ might consist of Modus Ponens together with the axioms
A typical proof would instantiate axiom A1 as P → ((Q→P)→P) and A2 as (P → ((Q→P)→P)) → ((P→(Q→P)) → (P→P)) and then apply Modus Ponens to these two instances to prove (P→(Q→P)) → (P→P). One more application of Modus Ponens to this and A1 then proves P→P. The proof itself consists just of the above propositions in the order shown, with our explanations removed. To avoid any possible ambiguity as to what depends on what it is customary to leave some indication of whether a theorem has arisen as an axiom instance or from applying modus ponens, and which axioms or theorems were used to that end.
Another form taken by propositional calculus is sequent calculus, which has two sorts, propositions as in ordinary propositional calculus, and pairs of lists of propositions called sequents, such as P∨Q, P∧R,… P, Q→R,…. The two halves of a sequent are called the antecedent and the succedent respectively. The customary metavariable denoting an antecedent or part thereof is Γ and for a succedent Δ; thus Γ,A Δ would denote a sequent whose succedent is Δ and whose antecedent is Γ with A appended after it. The antecedent is interpreted as the conjunction of its propositions, the succedent as the disjunction of its propositions, and the sequent itself as the entailment of the succedent by the antecedent. Entailment differs from implication in that it is either true or false even when the propositions are interpreted in some other Boolean algebra than the twovalued one. In this sense entailment is an external form of implication, meaning external to the Boolean algebra, thinking of the reader of the sequent as also being external and always interpreting antecedents and succedents as twovalued statements of fact, in contrast to their constituent propositions which are interpretable in any Boolean algebra. The natural interpretation of is as ≤ in the partial order of the Boolean algebra defined by x ≤ y just when x∨y = y. This ability to mix external implication and internal implication → in the one logic is among the essential differences between sequent calculus and propositional calculus.
Boolean algebra as the calculus of two values is fundamental to digital logic, computer programming, and mathematical logic, and is also used in other areas of mathematics such as set theory and statistics.
Digital logic codes its symbols in various ways: as voltages on wires in highspeed circuits and capacitive storage devices, as orientations of a magnetic domain in ferromagnetic storage devices, as holes in punched cards or paper tape, and so on. Now it is possible to code more than two symbols in any given medium. For example one might use respectively 0, 1, 2, and 3 volts to code a foursymbol alphabet on a wire, or holes of different sizes in a punched card. In practice however the tight constraints of high speed, small size, and low power combine to make noise a major factor. This makes it hard to distinguish between symbols when there are many of them at a single site. Rather than attempting to distinguish between four voltages on one wire, digital designers have settled on two voltages per wire, high and low. To obtain four symbols one uses two wires, and so on.
Programmers programming in machine code, assembly language, and other programming languages that expose the lowlevel digital structure of the data registers operate on whatever symbols were chosen for the hardware, invariably bit vectors in modern computers for the above reasons. Such languages support both the numeric operations of addition, multiplication, etc. performed on words interpreted as integers, as well as the logical operations of disjunction, conjunction, etc. performed bitwise on words interpreted as bit vectors. Programmers therefore have the option of working in and applying the laws of either numeric algebra or Boolean algebra as needed. A core differentiating feature is carry propagation with the former but not the latter.
Other areas where two values is a good choice are the law and mathematics. In everyday relaxed conversation, nuanced or complex answers such as "maybe" or "only on the weekend" are acceptable. In more focused situations such as a court of law or theorembased mathematics however it is deemed advantageous to frame questions so as to admit a simple yesorno answer—is the defendant guilty or not guilty, is the proposition true or false—and to disallow any other answer. However much of a straitjacket this might prove in practice for the respondent, the principle of the simple yesno question has become a central feature of both judicial and mathematical logic, making twovalued logic deserving of organization and study in its own right.
A central concept of set theory is membership. Now an organization may permit multiple degrees of membership, such as novice, associate, and full. With sets however an element is either in or out. The candidates for membership in a set work just like the wires in a digital computer: each candidate is either a member or a nonmember, just as each wire is either high or low.
Algebra being a fundamental tool in any area amenable to mathematical treatment, these considerations combine to make the algebra of two values of fundamental importance to computer hardware, mathematical logic, and set theory.
Twovalued logic can be extended to multivalued logic, notably by replacing the Boolean domain {0,1} with the unit interval [0,1], in which case rather than only taking values 0 or 1, any value between and including 0 and 1 can be assumed. Algebraically, negation (NOT) is replaced with 1 − x, conjunction (AND) is replaced with multiplication (xy), and disjunction (OR) is defined via De Morgan's law. Interpreting these values as logical truth values yields a multivalued logic, which forms the basis for fuzzy logic and probabilistic logic. In these interpretations, a value is interpreted as the "degree" of truth – to what extent a proposition is true, or the probability that the proposition is true.
The original application for Boolean operations was mathematical logic, where it combines the truth values, true or false, of individual formulas.
Natural languages such as English have words for several Boolean operations, in particular conjunction (and), disjunction (or), negation (not), and implication (implies). But not is synonymous with and not. When used to combine situational assertions such as "the block is on the table" and "cats drink milk," which naively are either true or false, the meanings of these logical connectives often have the meaning of their logical counterparts. However with descriptions of behavior such as "Jim walked through the door", one starts to notice differences such as failure of commutativity, for example the conjunction of "Jim opened the door" with "Jim walked through the door" in that order is not equivalent to their conjunction in the other order, since and usually means and then in such cases. Questions can be similar: the order "Is the sky blue, and why is the sky blue?" makes more sense than the reverse order. Conjunctive commands about behavior are like behavioral assertions, as in get dressed and go to school. Disjunctive commands such love me or leave me or fish or cut bait tend to be asymmetric via the implication that one alternative is less preferable. Conjoined nouns such as tea and milk generally describe aggregation as with set union while tea or milk is a choice. However context can reverse these senses, as in your choices are coffee and tea which usually means the same as your choices are coffee or tea (alternatives). Double negation as in "I don't not like milk" rarely mean literally "I do like milk" but rather convey some sort of hedging, as though to imply that there is a third possibility. "Not not P" can be loosely interpreted as "surely P", and although P necessarily implies "not not P" the converse is suspect in English, much as with intuitionistic logic. In view of the highly idiosyncratic usage of conjunctions in natural languages, Boolean algebra cannot be considered a reliable framework for interpreting them.
Boolean operations are used in digital logic to combine the bits carried on individual wires, thereby interpreting them over {0,1}. When a vector of n identical binary gates are used to combine two bit vectors each of n bits, the individual bit operations can be understood collectively as a single operation on values from a Boolean algebra with 2^{n} elements.
Naive set theory interprets Boolean operations as acting on subsets of a given set X. As we saw earlier this behavior exactly parallels the coordinatewise combinations of bit vectors, with the union of two sets corresponding to the disjunction of two bit vectors and so on.
The 256element free Boolean algebra on three generators is deployed in computer displays based on raster graphics, which use bit blit to manipulate whole regions consisting of pixels, relying on Boolean operations to specify how the source region should be combined with the destination, typically with the help of a third region called the mask. Modern video cards offer all 2^{23} = 256 ternary operations for this purpose, with the choice of operation being a onebyte (8bit) parameter. The constants SRC = 0xaa or 10101010, DST = 0xcc or 11001100, and MSK = 0xf0 or 11110000 allow Boolean operations such as (SRC^DST)&MSK (meaning XOR the source and destination and then AND the result with the mask) to be written directly as a constant denoting a byte calculated at compile time, 0x60 in the (SRC^DST)&MSK example, 0x66 if just SRC^DST, etc. At run time the video card interprets the byte as the raster operation indicated by the original expression in a uniform way that requires remarkably little hardware and which takes time completely independent of the complexity of the expression.
Solid modeling systems for computer aided design offer a variety of methods for building objects from other objects, combination by Boolean operations being one of them. In this method the space in which objects exist is understood as a set S of voxels (the threedimensional analogue of pixels in twodimensional graphics) and shapes are defined as subsets of S, allowing objects to be combined as sets via union, intersection, etc. One obvious use is in building a complex shape from simple shapes simply as the union of the latter. Another use is in sculpting understood as removal of material: any grinding, milling, routing, or drilling operation that can be performed with physical machinery on physical materials can be simulated on the computer with the Boolean operation x∧¬y or xy, which in set theory is set difference, remove the elements of y from those of x. Thus given two shapes one to be machined and the other the material to be removed, the result of machining the former to remove the latter is described simply as their set difference.
Search engine queries also employ Boolean logic. For this application, each web page on the Internet may be considered to be an "element" of a "set". The following examples use a syntax supported by Google.^{[1]}
"Search term 1" "Search term 2"
"Search term 1" OR "Search term 2"
"Search term 1" "Search term 2"

