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.[1]
The term comes from the Greek word tetras meaning "four".[2]
Tetrads are sometimes used by biologists for reporting the distribution of species to maintain a degree of confidentiality about their data,[3] 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.
| 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 |