In architecture, a floor is generally the lower horizontal surface of a room, but specially employed for one covered with boarding or parquetry (see also flooring). The various levels of rooms in a building are also called floors or stories, as "ground floor", "first story", "mezzanine floor", etc.
A confusion arises between American English and British English: In American English the floor at the ground level is the first floor, and the floor above is the second floor; whereas in British English, the floor at the ground level is the ground floor and the floor above is the first floor. However in either country, the first story is the floor at ground level.
The principal floor is the story which contains the chief apartments whether on the ground floor or the floor above; in Italy they are always on the latter and known as the piano nobile. The story below the ground floor is called the basement floor, even if only a little below the level of the pavement outside; the story in a roof is known as the attic floor. The expressions one pair, two pair, etc., apply to the storys above the first flight of stairs from the ground (see also carpentry).
Floors in buildings are often renamed or renumbered depending on the superstitions of the local population. In much of the European world, for example, the number 13 is considered unlucky (triskaidekaphobia), so the floor is renumbered to 12A or 12B, or even directly to 14. In Chinese culture, 4 is considered unlucky, so 4 would be renamed 3A or 3B, and further up the building, the floors would be numbered 12, 13, 13A, 15. Interestingly, this results in two floors numbered 13, whereas Europeans would prefer none!