The district includes 94 contributing sites along the Mason–Dixon line and includes the Transpeninsular Line, Post Marked West site, Tangent Line, the Arc Corner, and the Twelve-Mile Circle.

[2] The district was added to the National Register of Historic Places on February 18, 1975, for its significance in engineering and politics/government.

First, the southern boundary with Maryland along the Transpeninsular Line from Fenwick Island to the Midpoint Marker, the first crownstone set by Mason and Dixon.

Third, the northern boundary with Pennsylvania, the Twelve-Mile Circle.

In addition, the starting measurement point, the Post Marked West, and the boundary from the Arc Corner Monument to the Tri-State Monument are included in the district.