The radian (rad) is the SI unit of angle measurement, defined as the angle subtended by an arc equal in length to the radius of the circle. It is the natural unit for trigonometric functions in mathematics and physics — sin, cos, and tan all assume radian input. One radian equals approximately 57.296 degrees.

The gradian (gon), also called the grad, divides a right angle into 100 equal parts, making a full circle 400 gradians. It is used primarily in surveying and some European military mapping systems, where the base-100 system simplifies decimal calculations. One gradian equals exactly 0.9 degrees.