Mason-Dixon Line: Definition & History

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  • 0:00 Definition
  • 0:22 William Penn Creates…
  • 1:06 The Dispute
  • 1:59 Establishing the…
  • 2:55 Symbolism of the…
  • 3:37 Lesson Summary
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Lesson Transcript
Instructor: Amy Lively

Amy has an M.A. in American History. She has taught history at all levels, from university to middle school.

This lesson discusses the Mason-Dixon line. Learn more about the origins of the boundary that represents the symbolic border between the northern and southern United States, then test your knowledge with a quiz.


The term Mason-Dixon line refers to the boundary that separates Pennsylvania from Maryland and Maryland from Delaware. It's named for the surveyors who began work on establishing the borders in 1763 following a lengthy border dispute.

William Penn Creates Pennsylvania

King Charles II of England owed a debt; sixteen thousand pounds, to be exact. He owed the money to Sir William Penn, a British admiral who died in 1670. The King also was growing tired of the Quakers challenging the Church of England. So, to take care of both issues at once, in March of 1681 he gave the admiral's son, also named William Penn, 45,000 square miles of land in the New World. His debt was repaid and the Quakers were free to establish a new colony in America. Part of the land was conveniently located near the Delaware River. At the mouth of that river, Penn established the town of Philadelphia.The only problem was that Penn created Philadelphia in Maryland.

The Dispute

Cecilius Calvert, the Second Baron of Baltimore, was given a charter by King Charles I to establish a colony along the Atlantic Ocean. Calvert said that his charter included areas that we now know as Maryland, Delaware, and southern Pennsylvania. That meant that Philadelphia was on his land and he wanted to claim it as his own. That, of course, did not sit well with Penn and the two men argued about it for the rest of their lives. Their sons tried to come to an agreement after settlers got into a border conflict known as Cresap's War in 1730. The sons divided the disputed land in half in 1732, but the matter was still not settled until 1738 when King George II stepped in. Both colonies signed a treaty agreeing that the southern border of Pennsylvania was 15 miles south of Philadelphia.

Establishing the Mason-Dixon Line

In 1763, 81 years after the dispute between Penn and Calvert began, Charles Mason and Jeremiah Dixon arrived in Philadelphia. Mason, an astronomer, and Dixon, a surveyor, had worked together before, but establishing the 233-mile border between Maryland and Pennsylvania and the 83-mile border between Maryland and Delaware was quite a task. They worked their way north and west from the southwestern corner of present day Delaware using iron chains and a tool called a transit, which worked like both a telescope and a compass. Trigonometry helped them determine distances and angles. Huge stones from England, some weighing up to 600 pounds, were placed at every mile marker. It took five years of trudging through muddy fields and wading through creeks for Mason and Dixon and their team of laborers and Native American guides to finish the job.

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