Who Needs the Mason-Dixon Line?

In reading about Pennsylvania, as I have been doing a lot lately, I have come across many references to the Mason-Dixon line, the southern boundary of the colony and eventually the state. We’re all familiar with the Mason-Dixon line, of course. It has served as a powerful symbol of the division between the free North and the slaveholding South, the line enslaved people had to cross to escape to freedom. Despite that knowledge, it had never before occurred to me to wonder why the Mason-Dixon line exists at all. It’s quite a story, involving royal patronage, feuding families, a minor war, a famous lawsuit, and some cutting-edge science.