The levels of a multi-story building are numbered sequentially, from "basement", "one" or "ground" or "lobby" upwards. In some countries, the number 13 is considered unlucky and building owners will sometimes purposefully omit a floor so numbered. Hence, the 13th floor is occasionally given the number 14. Even landlords who are not themselves superstitious realize that the desirability of suites on a floor 13 might be compromised because of superstitious tenants, or commercial tenants who worry about losing superstitious customers. Based on an internal review of records, Dilip Rangnekar of Otis Elevators estimates that 85% of the buildings with elevators did not have a floor named the 13th floor. Future building designers, fearing a fire on the 13th floor, or fearing tenants superstitions about the rumour, decided to omit having a 13th floor listed on their elevator numbering. This practice became commonplace, and eventually found its way into mainstream culture and building design.  Also within the Disney ride The Twilight Zone Tower of Terror, this apparatus is known to have 13 floors, even though the dial on the elevator goes to 12, when on the elevator the doors have the numbers 13 on it, showing that the floor should not exist.
Most commonly, it is skipped altogether. The floor numbered 14 is in fact the thirteenth floor, and 13 is skipped altogether on the elevator console. Any calculations involving the height of a building based on the height of a floor will take this into account.
Sometimes the floor is simply renumbered as 12a or 12b; this does not affect the numbers of the higher floors.
Other buildings will often use names for certain floors to avoid giving a floor on the building the number 13 designation. One such example is the Radisson Hotel in Winnipeg, Manitoba, where the 13th floor is called the Pool floor.
Sometimes, the floor is put to some other use (see conspiracy theories, below).
Because the letter M is the 13th letter in the English alphabet, some people may use a letter M as a substitute for the floor numbered 13, such as 12, M, 14, and so forth. In Richmond, Virginia, the Monroe Park Towers has a 13th floor, but it is used for mechanical equipment and is only accessible from the freight elevator or the stairs. The M designation on the elevator buttons of the freight elevator can also be construed as meaning the "Mechanical" level in this particular building, or as a "Mezzanine" level.
Similarly, new buildings in some parts of China omit the fourth, fourteenth, twenty-fourth, etc. floors, as the word "four" (Hanzi: 四) sounds like "death" (死 – pronounced "sì" and "sǐ", respectively) in Mandarin, the predominant language for the country, and most other Chinese languages. A small number of buildings also follow the American tradition of omitting the thirteenth floor, with the fifteenth floor immediately following the twelfth.
In South Korea, buildings tend to include the fourth floor in spite of similar pronunciation issues in the Korean language, though some newer buildings may substitute the letter F in the place of the number 4.
Some conspiracy theorists have suggested that the thirteenth floor in government buildings is not really missing, but actually contains top-secret governmental departments, or more generally that it is proof of something sinister or clandestine going on. This implication is often carried over, implicitly or explicitly, into popular culture; for example in:
It is widely believed that Canary Wharf's One Canada Square houses a physical plant room on its level 13 but this is just another example of undeserved notoriety on the part of the 13th floor. One Canada Square's plant areas are in its basements (Levels B3 to M1) and above Level 50 (Level M2). The floor directly above level 12 is level 14.