A duplex is a residential building partitioned into two separate units, each with its own exterior-facing entrance. The units may be arranged side-by-side or, less frequently, stacked on top of each other. If both units share a common entrance, then it should be classified as a low-rise building.

The form of a duplex may closely resemble a single-family house or a small rowhouse cluster. A single-family house can sometimes be converted into a duplex by the addition of a full interior partition and a second main entrance.

Each of the two residential units has its own entrance facing outside. Side-by-side and stacked arrangements are both included; a shared main entrance instead meets the stated low-rise-building distinction.