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 A hectad is an area 10 km x 10 km square. The term has a particular use in connection with the British Ordnance Survey national grid, and then refers to any of the 100 such squares which make up a standard 100 km x 100 km myr ...

. 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.

References

Units of area {{measurement-stub