In the territory south of the Ohio River slavery became permanently established.

The river, therefore, became an extension of the original Mason and Dixon's Line with the new meaning attached: it became a division between free and slave territory.
It was apparently at first a mere matter of chance that a balance was struck between the two losses of States.

While Virginia remained a slave State, it was natural that slavery should extend into Kentucky, which had been a part of Virginia.

Likewise Tennessee, being a part of North Carolina, became slave territory.

When these two Territories became slave States, the equal division began.