The levels of a multi-story building are numbered sequentially, from "one" or "ground" upwards. In some countries, the number 13 is considered unlucky and building owners will sometimes purposely omit the thirteenth floor. Even landlords who are not themselves superstitious realize that the rentability of suites on the 13th floor might be compromised because of superstitious tenants, or tenants who are afraid their own customers might be superstitious.
How it is done
Simply being skipped
Most commonly, it is skipped altogether. Although the thirteenth floor has been skipped in terms of numbering (i.e., for hotel/apartment/suite numbers), the floor numbered 14 is technically the thirteenth floor of the building; it is simply not numbered as such. Any calculations involving the height of a building based on the height of a floor should take this into account (particularly in reference to BASE jumping).
Sometimes the floor is simply renumbered as 12a; 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 13th floor 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 also conspiracy theories, below). For instance, the thirteenth floor of One Canada Square houses the air conditioning equipment and no rentable offices, though the owners insist that this is merely an architectural coincidence.
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 - both are pronounced "sì" and "sǐ", respectively) in Mandarin, the predominant dialect for the country, and most other Chinese dialects. A small number of buildings also follow the Western tradition of omitting the thirteenth floor, with the fifteenth floor immediately following the twelfth.
Although the Hanja for four and death are read identically in Korean, buildings in South Korea tend not to omit the fourth floor. However, newer buildings tend to label the fourth floor with the letter F, instead 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 the films The Thirteenth Floor, the hidden research and development labs of Network 23 in the television program 'Max Headroom', and the computer game Floor 13 by Virgin Interactive. In the sci-fi comedy Red Dwarf "floor 13" referred to a secret brig which was several decks high.
It should be noted that to place a floor between those accessible from an elevator, it is necessary to either take longer to travel between the neighboring floors, or accelerate, both of which would be noticed by the riders.