A minor arc is an arc of a circle with a measure of less than or equal to 180 degrees.

A major arc is an arc of a circle with a measure of greater than or equal to 180 degrees

The arbelos or "cobbler's knife" is a figure made up from 3 semi circles.

Arc length is a fraction of the circumference.

The measure of the angle made by intersecting chords is equal to the average of the measures of the subtended arcs.

If secants intersect outside the circle, the angle between the secants is equal to the difference of the measures of the subtended arcs divided by two.

Any point on the unit circle has the coordinates (cos(θ), sin(θ)).