- The boundary line between Pennsylvania and Maryland, as run (1764-1767) by two English astronomers named Charles Mason and Jeremiah Dixon, that, before abolition, defined part of the northern boundary of states in which slavery was permitted.
1776, Thomas Jefferson, The Virginia Constitution (Letter to Edmund Pendleton), Philadelphia:
- 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 assembly.
1779, George Bryan, chapter 1779, in William Bradford Reed, editor, Life and Correspondence of Joseph Reed: Military Secretary of Washington, at Cambridge, Adjutant-general of the Continental Army, Member of the Congress of the United States, and President of the Executive Council of the State of Pennsylvania., volume 2, Philadelphia: Lindsay and Blakiston, published 1847, George Bryan to President Reed, Baltimore, August 31st, 1779, page 134:
- The Virginia gentlemen offer to divide exactly the 40th degree with us, […] Perhaps we would be as well off with Mason and Dixon's line continued.
- The boundary between the free and slave states at the time of the American Civil War. Also alluded to in situations where political or cultural properties of the free and slave states are contrasted.
1833, Charles Augustus Davis, “Letter III: Cause of the sudden termination of the President's Eastern Tour—Dance at Downingville—Trying on the Gineral's coat (July 14, 1833)”, in Letters of Jack Downing, Major, Downingville Militia, Second Brigade, to His Old Friend, Mr. Dwight, of The New-York Daily Advertiser., New-York: Harper & brothers, published 1834, page 36:
- […] and he tell'd me Georgia would go for me, arter the Gineral, as soon as any north of mason and dickson.
1843, Charles Fenno Hoffman, John Holmes Agnew, “Editor's Table”, in The Knickerbocker; Or, New-York Monthly Magazine, volume 22, New-York: John Allen, page 185:
- The epistles are not dated far apart; and in the second, the writer, who dwelleth near 'Mason and Dixon,' descants upon the awful climate hereabout in the summer months.