A tetrad is an area 2 km x 2 km square. The term refers to any of the 25 such squares which make up a standard hectad. Tetrads are sometimes used by biologists for reporting the distribution of species to maintain a degree of confidentiality about their data, though the system is not in universal use. 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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Units of area {{measurement-stub