Why was the division between the Union and the Confederacy called the Mason Dixon line?

to finally resolve an 80 year old land dispute in the 1760s between the family who owned Maryland & the family who owned Pennsylvania & perhaps also Delaware at that time

& which had arisen owing to an inadvertent territorial overlap in the original 17th century proprietary charters granted to their respective ancestors by the king of England

not until 1820 & only then by coincidence did the same line come to have any significance with regard to the northern boundary of slavery

& it was extended westward in that new context til 1861

when it reached the southwest corner of kansas by most accounts

or perhaps it actually continued as far as the pacific coast at the Mexican border

insofar as California was a free state

but the mason dixon line of the 19th century is really an entirely different matter than the mason dixon line of the 18th century