Duplex
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.