The point about which an angle is measured is called the angle's vertex, and the angle 
associated with a given vertex is called the vertex angle.
In a polygon, the (interior, i.e., measured on the interior side of the vertex) are generally denoted 
or 
. The sum of interior angles in any
-gon is given by 
radians, or 
(Zwillinger 1995, p. 270).
