What Formed The Border Between Slave And Free States East Of The Mississippi River?

The Mason-Dixon Line and the Ohio River formed the border between slave and free states east of the Mississippi River.

The mason dixon line. An imaginary line running east and west, which runs through Ky. Which I believe was the border state, [union.] We have monuments to both confederate and union soldiers, since the state was mostly a southern sympathizer, but was on the union side.