Mason and Dixon's Line
The southern boundary-line which separated the free states of Pennsylvania from what were at one time the slave states of Maryland and Virginia. It lies in 3943' 26' north latitude, and was run by Charles Mason and Jeremiah Dixon, two English mathematicians and surveyors (between November 15th, 1763, and December 26th, 1767).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Mason and Dixon's Line from Infoplease: