A historic structure is a home or building deemed significant because of its political, social, or architectural importance in a particular historical period. In order to be legally designated as a historic structure, the building or home must achieve historic certification by the U.S. Secretary of the Interior, be listed in the National Register of Historic Places, or be registered within a historic district. Historic districts are groupings of designated areas that contain a large number of historic structure residences, buildings or sites. Historic structure owners must usually adhere to special alteration and maintenance rules concerning the property. Owners of historic structures are also currently entitled to a 20 percent tax credit for the physical restoration of their properties when those restoration efforts are in accordance with national, state and regional historic structure laws.