A three-decker, triple-decker triplex or stacked triplex, in the United States, is a three-story (triplex) apartment building. These buildings are typically of light-framed, wood construction, where each floor usually consists of a single apartment, and frequently, originally, extended families lived in two, or all three floors. Both stand-alone and semi-detached versions are common.

A duplex house plan has two living units attached to each other, either next to each other as townhouses, condominiums or above each other like apartments. By contrast, a building comprising two attached units on two distinct properties is typically considered semi-detached or twin homes but is also called a duplex in parts of the Northeastern United States, Western Canada, and Saudi Arabia.