An angle bisector divides an angle into two congruent angles. If the original angle is $heta$, each part measures $heta / 2$.

That makes the bisector useful in compass-and-straightedge constructions, triangle incenter work, and quick checks in geometry problems.

A $72^circ$ angle splits into two $36^circ$ angles.

The highlighted dashed ray in the preview is the bisector itself, so you can compare the two equal parts at a glance.