It was a mere accident that the line drawn by Mason and Dixon between Pennsylvania and Maryland became known in later years as the dividing line between slavery and freedom. The six States south of that line ultimately neglected or refused to abolish slavery, while the seven Northern States became free. Vermont became a State in 1791 and Kentucky in 1792. The third State to be added to the original thirteen was Tennessee in 1796. At that time, counting the States as they were finally classified, eight were destined to be slave and eight free. Ohio entered the Union as a State in 1802, thus giving to the free States a majority of one. The balance, however, was restored in 1812 by the admission of Louisiana as a slave State. The admission of Indiana in 1816 on the one side and of Mississippi in 1817 on the other still maintained the balance: ten free States stood against ten slave States. During the next two years Illinois and Alabama were admitted, making twenty-two States in all, still evenly divided. The ordinance for the government of the territory north of the Ohio River, passed in 1787 and reenacted by Congress after the adoption of the Constitution, proved to be an act of great significance in its relation to the limitation of slavery. By this ordinance slavery was forever prohibited in the Northwest Territory. 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. There was yet an abundance of territory both north and south to be taken into the Union and, without any special plan or agitation, States were admitted in pairs, one free and the other slave. In the meantime there was distinctly developed the idea of the possible or probable permanence of slavery in the South and of a rivalry or even a future conflict between the two sections.