The radian is a unit of plane angle. It is represented by the symbol "rad" or, more rarely, by the superscript c (for "circular measure"). For example, an angle of 1.2 radians would be written "1.2 rad" or "1.2c ". The radian was formerly an SI supplementary unit, but this category was abolished from the SI system in 1995. For measuring solid angles, see steradian. Nowadays, radian is the de facto unit of plane angles for mathematicians, and the symbol "rad" is usually omitted in mathematicial writings. When using degrees, the ° symbol is used to distinguish it from radians.