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Tetrads are 2km by 2km squares. There are 25 tetrads in each 10-km square and each is identified by the grid reference of the 10-km square (e.g. SK45) plus a single letter suffix to identify the tetrad (e.g. SK45B). Tetrad codes run from A to Z (excluding O) from the SW (bottom left) corner to the NE (top right) corner as shown in the grid example.
