This finished what the Founding Fathers had started but could never quite agree on. Slavery was an economic system in the 18th century and they knew they wouldn't get the southern states to ratify the constitution if there were a prohibition against slavery in it. Slavery was not unique to the southern states in the late 18th century but by the mid 19th century, most northern states had outlawed it. The Emancipation Proclamation freed the slaves, this went a step further and made the practice illegal.
