In some places, the word "duplex" is more specific: In some regions, a "duplex" is a building with one unit above the other, separated by a floor and ceiling, but the staircase for upstairs residents is almost always indoors. In other regions, a "duplex" is a twin house: The units are next to each other, separated by a wall. In some regions, the word "duplex" is used for both kinds of buildings. The word "duplex" is not used in some cities, particularly Detroit, Dallas or Chicago.
Some duplexes have basements. Others do not.