Legal claim to the land was finally established by Lord Culpeper's grandson, Thomas Fairfax, 6th Lord Fairfax of Cameron, who became well known in the colony as "Lord Fairfax", in a survey authorized by Governor William Gooch in 1736. The lands of Lord Fairfax (and Northern Virginia) were defined as that between the Rappahannock and Potomac rivers, and were officially called the "Northern Neck". In 1746 a back line was surveyed and established between the headwaters of the Potomac and Rappahannock rivers, defining the west end of the grants. According to documents held by the Handley Regional Library of the Winchester–Frederick County Historical Society, the grants contained . They included the 22 modern counties of Northumberland, Lancaster, Westmoreland, Stafford, King George, Prince William, Fairfax, Loudoun, Fauquier, Rappahannock, Culpeper, Madison, Clarke, Warren, Page, Shenandoah, and Frederick counties in Virginia, and Hardy, Hampshire, Morgan, Berkeley, and Jefferson counties in West Virginia.