lied too ...imagine that

The M-D line is the state line between PA and WV it also cuts the northern panhandle off of WV

 

However, the original Mason-Dixon Line was actually a demarcation (or border) line between Pennsylvania and Maryland, in an effort to settle an 80-year land dispute between the two colonies. It also included the western border of present-day Delaware, as it was then a part of the Pennsylvania colony.

Over fifty years later, the boundary between the two states along the Mason-Dixon line came into the spotlight with the Missouri Compromise of 1820. The Compromise established a boundary between the slave states of the south and the free states of the north (however its separation of Maryland and Delaware is a bit confusing since Delaware was a slave state that stayed in the Union).
This boundary became referred to as the Mason-Dixon line because it began in the east along the Mason-Dixon line and headed westward to the Ohio River and along the Ohio to its mouth at the Mississippi River and then west along 36 degrees 30 minutes North.