Why are the Southern United States called “Dixieland”?

The nickname “Dixieland” didn’t come from the Mason-Dixon Line,
the boundary between the free and the slave states. Rather it’s from the
word dixie, which was what southerners called a French ten-dollar bank
note of New Orleans that was already in use in 1859 when Daniel
Emmet, a northern black man, wrote and introduced his song “Dixie,”
which spread the South’s nickname and somehow became a battle song
for the Confederacy.