Unicode is today gradually replacing the older ASCII derivatives limited to 7 or 8 bit codes.
The purpose of using plain text today is primarily a "lowest common denominator" independence from programs that require their very own special encoding or formatting (with due sacrifices and limitations). Plain text files can be opened, read, and edited with most text editors. Examples include Notepad (Windows), edit (DOS), ed, emacs, vi, vim, Gedit or nano (Unix, GNU/Linux), SimpleText (Mac OS), or TextEdit (Mac OS X). Other computer programs are also capable of reading and importing plain text. It can also be used by simple computer tools such as line printing text commands like
type (DOS and Windows) and
Plain text files are almost universal in programming; a source code file containing instructions in a programming language is almost always a plain text file. Plain text is also commonly used for configuration files, which are read for saved settings at the startup of a program.
Text was once commonly encoded in ASCII, using 8 bits for one letter or other character, encoding 7 bits, allowing 128 values, and using the 8th as a checksum bit when transferring a file. This just allowed the ordinary Latin alphabet, transfer control codes, parentheses and interpunction, which annoyed computer users, especially Portuguese and Swedish users. Therefore, when data transfer became more stable, the remaining 128 values were encoded everywhere differently, and in a way that made multilingual texts impossible to encode. At last Unicode was defined, which currently allows for 1,114,112 code values used for any modern text writing system, and a lot of extinct ones. For example, Unicode codes Chinese, Hebrew, and Cyrillic as well as Latin. Some of these text formats may be quite complicated to process correctly, but they still contain no structural data, such as bold start and end markers, and are therefore plain text.
The ASCII codes before
20H) are not intended as displayable characters, but instead as control characters. They are used for diverse interpreted meanings. For example, the code
0, sometimes denoted
Ctrl-@) is used as string end markers in the programming language C and successors. Most troublesome of these are the codes
LINE FEED =
CARRIAGE RETURN =
0DH). Windows and OS/2 require the sequence
CR,LF to represent a newline, while Unix and relatives use just the
LF, and Classic Mac OS (but not Mac OS X) uses just the code
CR. This was once a slight problem when transferring files between Windows and Unices, but today most computer programs treat this seamlessly.