Multifamily residential (also known as multidwelling unit or MDU) is a classification of housing where multiple separate housing units for residential inhabitants are contained within one building or several buildings within one complex. Units can be next to each other (side-by-side units), or stacked on top of each other (top and bottom units). A common form is an apartment building. Sometimes units in a multifamily residential building are condominiums, where typically the units are owned individually rather than leased from a single apartment building owner. Many intentional communities incorporate multifamily residences, such as in cohousing projects.