U.S. Constitution: Article I, Section 8, Clause 17
(governance of the District of Columbia)
Congress shall have the Power:
To exercise exclusive Legislation in all Cases whatsoever, over
such District (not exceeding ten Miles square) as may, by Cession
of particular States, and the Acceptance of Congress, become the
Seat of the Government of the United States, and to exercise like
Authority over all Places purchased by the Consent of the
Legislature of the State in which the Same shall be, for the
Erection of Forts, Magazines, Arsenals, dock-Yards, and other
needful Buildings;