Grid ref letters

The starting point was a large military grid system lying over Britain and its surrounding waters. It’s composed of 25 500km squares lettered from A-Z, excepting I. The British Isles lie on squares

H, N, S and T. This is the first letter in a grid reference.

Each of these 500km squares is then subdivided into 100km by 100km squares, again lettered A-Z, excluding I. This is the second letter in a grid reference.

So, although N falls over most of Scotland and northernmost England, and S over central and west England, they don’t stand for North or South – in case you’d ever wondered.