Arc

An arc is part of the curved circumference between two points. Those two points divide the circumference into two arcs; when their lengths differ, the shorter is the minor arc and the longer is the major arc. These are curved lengths, not straight chords.

When two arcs are drawn with the same compass opening, their intersections reveal points that are equal distances from the arc centres. This is what makes the constructions accurate. Leave the arcs in place so the reasoning behind the finished line can be seen.

Choose a convenient width and draw an arc across both arms of the angle.