The Three-Fifths Compromise was an agreement that was reached during the Constitutional Convention of 1787, in which three-fifths of the enslaved population in each state would be counted as population for the purposes of representation in the House of Representatives and for taxation. This was controversial because it essentially allowed slaveholding states to have greater representation in Congress due to their larger populations, even though a significant portion of their population was not granted the full rights of citizenship. The compromise was ultimately included in Article I, Section 2, Clause 3 of the Constitution, but it was abolished by the 13th Amendment in 1865.
