See state maps here showing when counties were formed over time. Go to 'County Boundary Changes for Each Year' about half way down the page. North Carolina Click Rotating Formation button
Virginia Click Rotating Formation button
With a little work, this map can show counties in two different states at the same time. E.g., if you're interested in seeing Guilford Cnty NC and Grayson Cnty VA and their relative positions.
Problem: View relative position of Guilford Cnty NC & Grayson Cnty VA
Solution: Go to site above and in “Find location:” box enter “Guilford County NC” then enter “Grayson County VA”
Grayson County was formed in 1793 from part of Wythe County and was named for
William Grayson, one of Virginia's first two senators. and a part of Patrick County
was added in 1810. Note that the eastern half of the county became Carroll county in 1842. http://www.myvirginiagenealogy.com/va_county/gra.htm
Carroll
Carroll County was formed in 1842 from Grayson County and was named for Charles Carroll of Carrollton, a signer of the Declaration of Independence from Maryland. And part of Patrick County was added in 1856. The county seat is Hillsville.