Just one day out from polling day, the 2024 presidential election is too close to call. The outcome will be decided in just a handful of swing states, of which Pennsylvania is perhaps the most coveted by both candidates.

Pennsylvania is a real cross-section of American society and the northeastern state is home to a key historical dividing line.

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The Mason-Dixon Line was surveyed from 1763 and 1767 by two Britons, Charles Mason and Jeremiah Dixon, who gave it their name. The 233-mile border was introduced to help settle a land dispute between proprietors of Pennsylvania and Maryland but it became a key factor in another, more infamous issue in US history.

What does the Mason-Dixon Line have to do with slavery?

Despite originated as a dispute-settling boundary between two states, the Mason-Dixon Line would go on to form a key dividing line in the pre-Civil War period. The extended Mason-Dixon Line, combined with the Ohio River, was used as the boundary between the southern slave-owning states and the north, where the practice of slavery was outlawed.

The Missouri Compromise of 1820 required a border to be established between the free states and the slave states, with slavery outlawed above the 36° 30′ parallel. That is roughly between Tennessee and Kentucky.

However some states above that line also objected to slavery, meaning that the Mason Dixon line became the informal boundary between the two halves of the United States. That probably gave rise to the names “Dixie” or “Dixieland” used to describe the southern parts of the United States.

Any hopes that the border would help to bridge a peace between the north and south were (relatively) short-lived. The Missouri Compromise ensured a temporary accord but a few decades later it had collapsed and the United States was embroiled in civil war, eventually bringing an end to slavery.