Tetrad (area)
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.
The term comes from the Greek word tetras meaning "four".
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 according to the "DINTY" system as shown in the grid below, which takes its name from the letters of the second line.
| E | J | P | U | Z |
| D | I | N | T | Y |
| C | H | M | S | X |
| B | G | L | R | W |
| A | F | K | Q | V |