A triangle has three vertices. A triangle consists of three lines, and the location where one line endpoint meets another line endpoint is called a vertex. A square consists of four vertices, while an octagon contains eight vertices.

Any polygon has the same number of vertices as it does sides; this is a constant that never changes, no matter how many lines are introduced into a polygonal shape.

A collinear vertice refers to a set of points that lie on the same straight line.

Triangles are classified by the length of their sides and by the interior angles of the corners.