‘The term "duplex" can also be extended to three-unit and four-unit buildings, or they can be referred to with specific terms such as triplex and fourplex or quadplex/quadruplex. Because of the flexibility of the term, the line between an apartment building and a duplex is somewhat blurred, with apartment buildings tending to be bigger, while duplexes are usually the size of a normal house.
In dense areas like Manhattan and downtown Chicago, a duplex or duplex apartment refers to a maisonette, a 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.’
http://en.wikipedia.org/wiki/Duplex_(building)
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