When William Penn petitioned the King for his charter for the Colony of Pennsylvania in 1681, the western boundary was described as being "...on the west limited as Maryland is..." Maryland was then limited by the extent of the Potomac River whose head spring was not finally settled until Lord Fairfax's survey of 1736 and resurvey of 1748. However, when Penn finally received his charter the bureacrats in London set the western boundary at five degrees of longitude west of the point where the east end of the southern boundary crosses the Delaware River. This proved a serious problem for Pennsylvania.
The first part of the problem was that the southern boundary had erroneously been placed below the northern boundary of Maryland. However, even when that problem was finally negotiated by Penn and Lord Baltimore, the western boundary was still unknown. Where the western boundary of Maryland was set at a particular physical feature that was finally found and marked (the first spring of the Potomac River), the western boundary of Pennsylvania had to be measured five degrees west from a point on the eastern boundary. Up to the time when Col. George Washington fired the first shots of the French and Indian War at Jumonville glen and Ft. Necessity, no one knew exactly where Pennsylvania's western boundary was.
When Mason and Dixon surveyed the Maryland-Pennsylvania boundary after the end of the French and Indian War, they could not continue the line the required five degrees of longitude west of the Delaware River. The local Indians refused to let white men with the survey instrument that they thought would "steal the Indian's land" pass westward beyond a branch of the Catawba Path.
It was not until 1784 after we had won our Independance, that Virginia and Pennsylvania finally agreed to find and set the present western boundary at five degrees of longitude west of the Delaware River where it bisects Pennsylvania's southern boundary. The marker was placed by accurately determining the location by astronomical observations during the Autumnal equinox of 1784. That westward location of the boundary placed both Fort Necessity and the site of Fort Duquesne (by then the city of Pittsburg) in the State of Pennsylvania. On the well known Fry and Jefferson map of the colony of Virginia drawn in 1751-1754, the boundary had been shown as placing Fort Necessity in Pennsylvania and Ft. Duquesne in Virginia. That is why one can truly say that the French and Indian War was started by Virginians in what they thought was Virginia; today we recognize the locations to be in Pennsylvania.