Apothem

An apothem of a regular polygon is a line segment from the center of the polygon to the midpoint of one of its sides. An apothem is always perpendicular to the side that contains the midpoint. Since the midpoint of each side of a regular polygon is intersected by the incircle, an apothem is a radius of the incircle. The sagitta is the line segment from the edge of the regular polygon to the edge of the circumcircle of the regular polygon that is collinear with the apothem.

The apothem of a chord of a circle is a line segment from the center of the circle to the midpoint of a chord of the circle. The sagitta is the line segment from the midpoint of the chord to the edge of the circle that is collinear with the apothem. Together, the sagitta and the apothem make a radius of the circle.