gradian
Angleexact by definition
The gradian, also called the gon or grade, is exactly 1/400 of a full turn, so a right angle is exactly 100 gon. In radians that is \(\pi/200\), an exact definition whose decimal value must be rounded. It is not an SI unit.
Watch out: The abbreviation "grad" is easily confused with "gradient" and with the calculator label GRA, which is the source of most accidental use. A calculator left in GRA mode gives sin(100) = 1 instead of the expected degree answer.
| 1 gon | 0.9 ° |
The gradian comes from the French Revolution's programme of decimalising everything, which produced a circle of 400 parts alongside the decimal calendar and decimal time. Unlike those, it partly survived: it is still used in surveying in France and some neighbouring countries, and most surveying calculators offer a GON mode.