angle initial side

The initial side of an angle θ drawn in angle standard position is the side lying on the positive x-axis. In general, viewing an angle as the amount of rotation about the intersection point (the vertex) needed to bring one of two intersecting lines (or line segments) into correspondence with the other, the line (or line segment) being rotated about the vertex is thought of as the initial side.

angle | angle standard position | intersection | line | line segment | rotation | terminal side | vertex