The grid square convention 1

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To reduce the number of digits required to give a National Grid reference, a system of grid squares denoted by letters is used. The National Grid is broken down into progressively smaller squares identified by letters and then by numbers.

The largest units are 500 km by 500 km squares, each square designated by a prefix letter - the first prefix letter to be quoted in the National Grid reference. This grid is actually larger than the National Grid because it was adopted from a military grid system that was already in use.

Note: the 25 letters are the letters of the alphabet, excluding the letter 'I'.