Washington, D.C., the capital of the United States, has a unique and diverse architectural history. Encompassing government, monumental, commercial, and residential buildings, D.C. is home to some of the country's most famous and popular structures designed by some of the leading architects of their time. The popularity of Washington's buildings is evident by the fact that a 2007 poll of Americans by the American Institute of Architects found that six of the top 10 most popular U.S. structures were located in Washington, D.C. Overall, 17 of the top 150 most popular structures were located in the capital.