Mason–Dixon Line – The royal charters that established MD and PA and granted them to the Calvert and Penn families did not agree as to where boundaries lay. The British court had to declare a border, so a proper survey was needed. Surveyors in the colonies were no match for the difficult job, so 2 experts from England, Charles Mason and Jeremiah Dixon, an astronomer and a surveyor,spent from 1763-1767 surveying the N/S line now separating DE and MD, and the E/W line dividing MD and PA. Stones engraved with a “P” were placed on the northern side and an “M” on the southern. Each 5th mile was marked with a crownstone carved with the coat of arms of William Penn and Lord Baltimore.

Fifty years later, in the Missouri Compromise of 1819-1820 (covering the slavery issue in new territories admitted to the Union), an imaginary line along the MD-PA border and beyond separated the free States from the slave States. Called the Mason-Dixon line, it came to be known as the boundary between the North and South.