You may have noticed that there are buttons on your scientific calculator labelled "DEG", "RAD", and "GRAD". You may have wondered, what do they mean?

A *degree* (the usual symbol is **°**.) is
^{1}⁄_{360} of the angle of a circle. A right angle is 90°.

A *radian*is ^{1}⁄_{2π} of a circle, so 1 rad
is approximately equal to 57.296°. If you take two radii of a circle with
radius 1 where the angle measurement between the two is 1 rad, the length of
the arc of the circle with endpoints being the endpoints of the radii will be
1 as well.
A *gradian* is
sometimes known as a metric degree. A right angle (90°) is defined as
100 gradians, so a full circle
(360°) would be 400 gradians. Gradians are not as commonly
used as degrees or radians are.