National Grid index diagrams

1:25,000 First and Second Series maps : How to find a number for the sheet you want
We use a 1980s road atlas as a useful and detailed index to these series (the First Series and the Second Series (Pathfinder)).
Look in the front of any road atlas, and if it mentions using the National Grid, then one small square on the page is one 1:25,000 First Series map.The maps are numbered using two letters, followed by two digits. Consider Haywards Heath, just north of Brighton. Somewhere on the page will be two large letters TQ.
Haywards Heath is in the centre of a square, and the map is TQ 32. The 3 is the digit for the vertical line forming the left edge of the square, and can be seen at the top and bottom of the line, on the edge of the page. The 2 is the digit for the horizontal line forming the bottom edge of the map, and can be seen on the left and right of the atlas pages.
If you already own a map from this series, compare it to a road atlas and things usually become clear.

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