The units in structure data capture the proportion of the housing structures with predetermined numbers of units. According to the US Census Bureau, a structure is a separate building which either has open space on all four sides or is separated from other structures by dividing walls that extend from ground to roof. In double houses, row houses, and houses attached to nonresidential structures, each building is a structure if the common wall between them goes from ground to roof. Sheds and private garages which adjoin houses are not counted as separate structures. In apartment developments, each building with open space on all sides is considered a separate structure. The count of housing units in a structure is the total number of units in the structure, both occupied and vacant units. In the tabulations, occupied mobile homes or trailers, tents, and boats are included in the category one housing unit in structure.