The Congress shall have 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… (The U.S. Constitution, Article I, Section 8, Clause 17) https://www.heritage.org/report/the-constitution-and-the-district-columbia