Floor numbering in a building can cause misunderstandings between speakers of different varieties of the English language.
In most varieties, descending from English as spoken in the United Kingdom, the floor of a building which is entered at ground level from the outside is termed the ground floor, and the next floor up is the first floor. However in American English the floor at ground level is termed the first floor and the next floor up is the second floor.
Imagine that you have entered the Jefferson Building through the west ground level entrance, under the granite archway.
A labeled map of the GroundFloor of the Jefferson Building is available.
On GroundFloor of the Thomas Jefferson Building you will also find a Sales Shop to the left as you enter the building, a cloakroom beyond the security stations, and the Visitors' Theater where you may see a short film about the Library of Congress.
The principal floor is the storey which contains the chief apartments, whether on the groundfloor or the floor above; in Italy they are always on the latter and known as the piano nobile.
The storey below the groundfloor is called the basement even if only a little below ground level, or the cellar; the story in a roof is known as the attic or the loft.