Gradians per Degree

The gradian, or gon, divides the right angle into 100 parts instead of 90, making a full circle 400 gon and the conversion an exact 10/9. It was introduced during the French Revolution alongside decimal time and the decimal calendar, part of a wholesale attempt to rationalise measurement that succeeded spectacularly for length and mass and failed almost completely for angle and time.

Unlike decimal time, the gradian did not quite die. Continental European surveying kept it, because a quadrant of 100 gon meshes with decimal subdivision and because a gon of latitude is conveniently 100 km — the metre having been defined in 1791 as one ten-millionth of the quarter meridian, which makes the whole meridian 10000 km and 100 gon. Most scientific calculators still carry a GRAD mode next to DEG and RAD, and the commonest use of that key is pressing it by accident and wondering why the answers are wrong.