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. The term "duplex" is not extended to three-unit and four-unit buildings, as they would be referred to with specific terms such as three-family (or triplex) and fourplex (or quadplex/quadruplex) or a more general multiplex. 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 single-family house. Variants US 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. In this context, an apartment locating on one floor only may be called a simplex. UK In the United Kingdom, the term duplex is sometimes used by property professionals such as architects and estate agents and refers only to a flat or apartment on two floors connected by an inner staircase though many newer apartments have open-plan designs including mezzanines. The far more commonly used term is 'maisonette' meaning two houses split horizontally like a flat but with its own separate door unlike flats that would have an initial shared door. Canada While in most of the world, a Duplex is described by many as an identical two-unit dwelling connected by a common wall. In Canada, this description refers to Single-Family Attached Homes that could be sold separately and owned by different owners. Most real estate agents define a Duplex as a residential property with rental components, not necessarily identical. Most real estate investors take advantage of investing in duplexes, triplexes, or multi-family by generating equity on the property while tenants pay the mortgage. Australia In Australia, a duplex is often considered a residential building containing two homes that share a common central wall. As