"On this day in 1767, Charles Mason and Jeremiah Dixon
complete their survey of the boundary between the colonies
of Pennsylvania and Maryland as well as areas that would
eventually become the states of Delaware and West Virginia.
The Penn and Calvert families had hired Mason and Dixon,
English surveyors, to settle their dispute over the boundary
between their two proprietary colonies, Pennsylvania and
Full (well, maybe) history lesson here and here.
Musical accompaniment here: