A duplex house is a two-unit apartment building or condominium, usually indistinguishable from a normal house on the exterior. Duplexes are especially common in the Northeastern United States and urban areas throughout the United States. Duplex can also refer to three- and four-unit buildings, the words triplex and quadruplex most often refer to movie theaters. The line between an apartment building and a duplex is therefore somewhat blurred, with apartment buildings tending to be bigger, while duplexes are usually the size of a normal house.

Especially in dense areas like Manhattan, a duplex apartment refers to single dwelling unit spread over two floors connected by an indoor staircase. Similarly, a triplex apartment refers to an apartment spread out over three floors. These properties can be quite expensive, and include the most expensive property in Manhattan as of 2006 (according to Forbes Magazine), a triplex atop The Pierre Hotel.