I'm continually surprised at how many people don't understand the three-fifths compromise in the original U.S. Constitution, usually describing it somewhat like Cord Jefferson does: "the three-fifths compromise, in which the government decided that black slaves were subhuman." The clear implication here that the Constitution codified a black slave was worth only 60% of a normal human, because they didn't count as much as "free Persons" in establishing proportional representation in the House.
But this understanding is completely backwards; black slaves would have been better off if the Constitution counted them at one-fifth, or not at all. The southern states would have been much happier had the slaves counted as whole persons, or better yet, 5 persons each!
Alas, subsequent history shows that those fears were hardly misplaced. As we all know, "radical" reconstruction failed, blacks were disenfranchised, and for at least the next century precisely what the Radicals feared in fact took place: the racist southern political class would continue to punch far above its national weight until at least the Voting Rights Act of 1965, and arguably until 2009.
The three-fifths compromise was, from a purely practical perspective, a positive inasmuch as it weakened the South relative to the North. But it was hugely negative from an ideological perspective because it established in America’s founding document that slaves were not analogous to women and children – that they were something less than full (nonvoting) members of the community.