## The angle and its elements

Two straight lines that are not contained in the same line, and have the same origin, divide the plane into two regions: one convex and one non-convex.

Each of these regions, together with the semi-straight ones, form a **angle**. Thus, the two straight lines determine two angles:

Every angle has **two sides **and **a vertex**. The sides are the straight lines that determine it. The vertex is the common origin of these semi-straights.

The convex angle of vertex O and sides is indicated by: AÔB, BÔA or Ô.