The line of latitude that corresponded with Missouri's Southern border.
That was the Missouri Compromise of 1820, which kept the peace for thirty years, until the admission of California, which extended too far either side of the line.
This led to the Compromise of 1850 - a new deal, where there was no single line of demarcation.
I believe Mississippi
Missouri Compromise
Mississippi was a slave state until the end of the Civil War.
To avoid arguments over which Western territories could be admitted as slave-states. Any new state North of Missouri's Southern border would be free soil.
It was the border between Pennsylvania (free soil) and Maryland (slave-state). Beyond that, it had no actual significance.
The Missouri River
The Missouri River
The Missouri River
The Mississippi River.
This latitude followed the Southern border of the new state of Missouri. It was taken as the parallel that would mark the divide between slave-states and free soil, in the rest of the lands acquired from France in the Louisiana Purchase. It kept the peace for thirty years, until the huge territories acquired from Mexico rendered it obsolete.
It was the southern border of Missouri and was just south of the point where the Ohio River ends.
I believe Mississippi
Missouri Compromise
Slave ownership in territories could be decided by popular sovereignty
Mississippi was a slave state until the end of the Civil War.
The "Border States" were slave states.
The border between slave states and free states