Mason And Dixon's Line, the parallel of lat. 39° 43' 26.3" N., which separates Pennsylvania from Maryland, drawn by Charles Mason and Jeremiah Dixon, two distinguished English mathematicians and astronomers. As the northern limit (with the exception of small portions of Delaware and Virginia) of the original slave states, it was prominently mentioned in the controversies concerning slavery. It begins at the N. E. corner of Maryland, and runs due W. The years from 1681 to 1768 were marked with constant dissension and conflict bet ween the rival proprietaries of Pennsylvania and Maryland and their partisans, on the subject of 'their common boundary; and the vicinity of this line was the theatre of riot, invasion, and bloodshed. Mason and Dixon arrived in Philadelphia on Nov. 15, 1763, and commenced their work in December, which was continued to a point 244 m. from the Delaware river, and within 36 m. of the whole distance to be run, where they were compelled to suspend operations in consequence of opposition by the Indians. They returned to Philadelphia, and were discharged on Dec. 26, 1767. At the end of every fifth mile a stone was planted, graven with the arms of the Penn family on one side, and of Lord Baltimore on the other.

The intermediate miles were marked with smaller stones having a P on one side and an M on the other. The stones were all sent from England. In November, 1782, Col. Alexander McClean of Pennsylvania and Joseph Neville of Virginia ran the remaining part of the line, which was tested and corrected by astronomical observations, and permanently marked, in 1784. In 1849 the former surveys were revised, and found correct in all important points.