The letter E would be at the vertex. The two lines enclosing the angle E would be ED and EF. Usually, to make it quite clear we would call angle E by the description "angle DEF
http://wiki.answers.com/Q/What_is_the_vertex_of_th...

The point about which an angle is measured is called the angle's vertex, and
http://www.chacha.com/question/whatisthedegree'...

1. Find the slopes of the lines that contains each segment. The slope of a line is calculated by dividing the change in ycoordinates by the change in xcoordinates: slope = (y2 
http://www.ehow.com/how_8443661_vertexangle.html

In Astrology, a vertex is defined as a theoretical point where the Sun's orbit around the earth crosses a vertical plane and divides the celestial sphere.
http://www.ask.com/webanswers/Science/Mathematics...
