Washington D.C. is the capital of the United States and home to important government buildings and monuments. It houses the three branches of government and memorials honoring presidents Lincoln and Jefferson. Notable structures include the Pentagon, Arlington National Cemetery, and the Tidal Basin. The city's original design was overseen by Charles L'Enfant, though Andrew Ellicott later took over. The Smithsonian Institution operates museums that preserve U.S. history and support research.