A tetrad is an area 2 km x 2 km square. The term has a particular use in connection with the British Ordnance Survey national grid, and then refers to any of the 25 such squares which make up a standard hectad.
The tetrads are labelled from A to Z (omitting O) according to the "DINTY" system as shown in the grid below, which takes its name from the letters of the second line.
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