What formed a border between slave and free states east of the Mississippi River What state was the exception?

The border between slave and free states east of the Mississippi River was largely defined by the Mason-Dixon Line and the Ohio River. This division was established during the early 19th century as states entered the Union. The exception to this border was Missouri, which was a slave state located north of the Ohio River, complicating the geographical and political landscape of slavery in the United States.