The three-fifths compromise was an agreement at the constitutional convention, which stated that in calculating population for state representation

Three out of five slaves in slave states would be counted in calculating the state's population.

Details:

The Three-Fifths Compromise was a way of accounting (somewhat) for the population of slaves in states that permitted slavery.  For taxation and representation purposes, the question was whether slaves should count in the population figures.  (They were not considered voting citizens at that time.)   The Three-Fifths Compromise said that three out of every five slaves could be counted when determining a state's population size for determining how many seats that state would receive in the House of Representatives.

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