The Mason-Dixon Line: What Is It? Where is it? Why is it Important?

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The British men in the business of colonizing the North American continent were so sure they “owned whatever land they land on” (yes, that’s from Pocahontas), they established new colonies by simply drawing lines on a map. 

Then, everyone living in the now-claimed territory, became a part of an English colony. 

Map of British territory in North America
A map of the British dominions in North America,  c1793.

And of all the lines drawn on maps in the 18th century, perhaps the most famous is the Mason-Dixon Line.

What is the Mason-Dixon Line?

Stargazer's stone
The “Stargazer’s Stone.” Mason and Dixon used this as a base point while plotting the Mason-Dixon line. The name comes from the astronomical observations they made there.

The Mason-Dixon Line is a boundary line that makes up the border between Pennsylvania, Delaware, and Maryland. Over time, the line was extended to the Ohio River to make up the entire southern border of Pennsylvania.

But it also took on additional significance when it became the unofficial border between the North and the South, and perhaps more importantly, between states where slavery was allowed and states where slavery had been abolished.

READ MORE: The History of Slavery: America’s Black Mark

Where is the Mason-Dixon Line?

For the cartographers in the room, the Mason-Dixon Line is an east-west line located at 39º43’20” N starting south of Philadelphia and east of the Delaware River.

For the rest of us, it’s the border between Pennsylvania, Virginia, Maryland, and West Virginia. 

Mason-Dixon Line Map

Take a look at the map below to see exactly where the Mason Dixon Line is:

Mason-Dixon Line

Why Is it Called the Mason-Dixon Line?

It is called the Mason-Dixon Line because the two men who originally surveyed the line and got the governments of Maryland, Pennsylvania, and Delaware to agree, were named Charles Mason and Jeremiah Dixon. 

A Plan of the West Line
“A Plan of the West Line or Parallel of Latitude” by Charles Mason, 1768.

The line was not called the Mason-Dixon Line when it was first drawn. Instead, it got this name during the Missouri Compromise, which was agreed to in 1820.

It was used to reference the boundary between states where slavery was legal and states where it was not. After this, both the name and its understood meaning became more widespread, and it eventually became part of the border between the seceded Confederate States of America and Union Territories.

Why Do We Have a Mason-Dixon Line?

In the early days of British colonialism in North America, land was granted to individuals or corporations via charters, which were given by the king himself.

However, even kings can make mistakes, and when Charles II granted William Penn a charter for land in America, he gave him territory that he had already granted to both Maryland and Delaware! What an idiot!?

Charles II of England
King Charles II of England.

But in his defense, the map he was using was inaccurate, and this threw everything out of whack. At first, it wasn’t a huge issue since the population in the area was so sparse there were not many disputes related to the border.

But as all the colonies grew in population and sought to expand westward, the matter of the unresolved border became a much more prominent in mid-Atlantic politics. 

The Feud

In colonial times, as in modern times, too, borders and boundaries were critical. Provincial governors needed them to ensure they were collecting their due taxes, and citizens needed to know which land they had a right to claim and which belonged to someone else (of course, they didn’t seem to mind too much when that ‘someone else’ was a tribe of Native Americans).

As a result, solving this border dispute became a major issue, and it became an even bigger deal when violent conflict broke out in the mid-1730s over land claimed by both people from Maryland and Pennsylvania. This little event became known as Cresap’s War.

Cresaps War
Map showing the area disputed between Maryland and Pennsylvania during Cresap’s War.

To stop this madness, the Penns, who controlled Pennsylvania, and the Calverts, who were in charge of Maryland, hired Mason and Dixon to survey the territory and draw a boundary line to which everyone could agree. 

But Mason and Dixon only did this because the Maryland governor had agreed to a border with Delaware. He later argued the terms he signed to were not the ones he had agreed to in person, but the courts made him stick to what was on paper. Always read the fine print!

This agreement made it easier to settle the dispute between Pennsylvania and Maryland because they could use the now established boundary between Maryland and Delaware as a reference. All they had to do was extend a line west from the southern part of Philadelphia, and…

The Mason-Dixon Line was born.

Later, in 1779, Pennsylvania agreed with Virginia to extend the Mason-Dixon Line west by five degrees of longitude to create the border between the two colines-turned-states (By 1779, the American Revolution was underway and the colonies were no longer colonies). 

In 1863, during the Civil War, West Virginia separated from Virginia and rejoined the Union, but the line remained as the border with Pennsylvania. 

It’s updated several times throughout history, the most recent being during the Kennedy Administration, in 1963.

The Mason-Dixon Line’s Place in History

At first glance, the Mason-Dixon Line doesn’t seem like much more than a line on a map. Plus, it was created out of a conflict brought on by poor mapping in the first place…a problem more lines aren’t likely to solve.

But despite its lowly status as a line on a map, it eventually gained prominence in American history and collective memory because of what it came to mean to some segments of the American population.

It first took on this meaning in 1780 when Pennsylvania abolished slavery. Over time, more northern states would do the same until all the states north of the line did not allow slavery. This made it the border between slave states and free states. 

Perhaps the biggest reason this is significant has to do with the underground resistance to slavery that took place almost from the institution’s inception. Slaves who managed to escape from their plantations would try to make their way north, past the Mason-Dixon Line. 

Underground Railroad map
Map of the Underground Railroad. The Mason-Dixon line drew a literal barrier between slave and free states.

However, in the early years of United States history, when slavery was still legal in some Northern states and fugitive slave laws required anyone who found a slave to return him or her to their owner, meaning Canada was often the final destination. Yet it was no secret the journey got slightly easier after crossing the Line and making it into Pennsylvania.

Because of this, the Mason-Dixon Line became a symbol in the quest for freedom. Making it across significantly improved your chances of making it to freedom. 

Today, the Mason-Dixon Line does not have the same significance (obviously, since slavery is no longer legal) although it still serves as a useful demarcation in terms of American politics. 

The “South” is still considered to start below the line, and political views and cultures tend to change dramatically once past the line and into Virginia, West Virginia, Kentucky, North Carolina, and so on. 

Beyond this, the line still serves as the border, and anytime two groups of people can agree on a border for a long time, everyone wins. There’s less fighting and more peace.

The Line and Social Attitudes

Because when studying the US history the most racist stuff always comes from the South, it’s easy to fall into the trap of thinking the North was as progressive as the South was racist. 

But this simply isn’t true. Instead, people in the North were just as racist, but they went about it in different ways. They were more subtle. Sneakier. And they were quick to judge Southern racist, pushing attention away from them.

In fact, segregation still existed in many northern cities, especially when it came to housing, and attitudes towards blacks were far from warm and welcoming. Boston, a city very much in the North, has had a long history of racism, yet Massachusetts was one of the first states to abolish slavery

As a result, to say the Mason-Dixon Line separated the country by social attitude is a gross mischaracterization. 

Mason-Dixon Crownstone Sign
Mason-Dixon Crownstone sign in Marydel, Maryland.

formulanone from Huntsville, United States [CC BY-SA 2.0

It’s true that blacks were generally safer in the North than in the South, where lynchings and other mob violence were quite common all the way up until the civil rights movement in the 1950s and 1960s.

But the Mason-Dixon Line is best understood as the unofficial border between the North and the South as well as the divider between free and slave states.

The Future of the Mason-Dixon Line

Although it still serves as the border of three states, the Mason-Dixon Line is most likely waning in significance. Its unofficial role as a border between the North and South only really remains because of the political differences between the states on each side. 

However, the political dynamic in the country is changing rapidly, especially as demographics shift. What this will do to the difference between North and South, who knows? 

Mason Dixon Line Trail
The “Mason Dixon Line Trail” stretches from Pennsylvania to Delaware, and is a popular attraction to tourists.

Jbrown620 at English Wikipedia [CC BY-SA 3.0

If we use history as a guide, it’s safe to say the line will continue to serve some significance if in nothing else except our collective consciousness. But maps are redrawn constantly. What’s a timeless border today can be a forgotten boundary tomorrow. History is still being written.


The Great Compromise of 1787

The Three-Fifths Compromise

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1 thought on “The Mason-Dixon Line: What Is It? Where is it? Why is it Important?”

  1. I’d say the line dividing the north and the south (culturally) is now south of Washington, DC. Fredericksburg, VA would be my opinion. Having lived, grown up, gone to school and now working in Northern, VA the attitudes here are much more in alignment with the north than the south.


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