Mason and Dixon line

Proper noun

the Mason–Dixon Line

1. (historical) The boundary line between Pennsylvania and Maryland, as run (1764–1767) by Charles Mason and Jeremiah Dixon, that, before abolition, defined part of the northern boundary of states in which slavery was permitted.
   • I am indebted to you for a topic to deny to the Pensylvania claim to a line 39 complete degrees from the equator. As an advocate I shall certainly insist on it; but I wish they would compromise by an extension of Mason & Dixon's line. — They do not agree to the temporary line proposed by our...1776, Thomas Jefferson, The Virginia Constitution (Letter to Edmund Pendleton), Philadelphia
2. The boundary between the free and slave states at the time of the American Civil War, or between states with different segregationist policies in the Jim Crow era.
   • […] and he tell'd me Georgia would go for me, arter^([sic]) the Gineral^([sic]), as soon as any north of mason and dickson^([sic]).1833, Charles Augustus Davis, “Letter III […] ”, in Letters of Jack Downing, Major […] , New-York: Harper & brothers, published 1834, page 36