Ordnance Survey National Grid
The system divides the map into squares. Each square is labelled with two letters. The first letter refers to squares that are 500 km by 500 km. Those squares are outlined in dark grey on the map to the right. Only four of these have a lot of land area in Great Britain: S, T, N and H. The O square contains a tiny area of North Yorkshire. Normally most of this area is submerged at high tide.