1. Radian (rad): The radian is a unit of plane angle. It may be defined as the angle between
two radii subtended at the centre of a circle by an arc equal in length to the radius.
2. Steradian (sr): The steradian is the unit of solid angle. It may be defined as the solid
angle, which, having its vertex in the centre of a sphere, cuts off an area of the surface of
sphere equal to that of a square having sides of length equal to the radius of the sphere.
The two supplementary units are :
| S.No. | Quantity | S.I. units | Symbol |
| 1. | Plane angel | radian | rad |
| 2. | Solid angel | steradian | sr |