vertex

A polygon vertex x_i of a simple polygon P is a principal polygon vertex if the diagonal [x_{(i − 1)},x_{(i + 1)}] intersects the boundary of P only at x_{(i − 1)} and x_{(i + 1)}. There are two types of principal vertices: ears and mouths.

 Ears

A principal vertex x_i of a simple polygon P is called an ear if the diagonal [x_{(i − 1)},x_{(i + 1)}] that bridges x_i lies entirely in P. (see also convex polygon)

 Mouths

A principal vertex x_i of a simple polygon P is called a mouth if the diagonal [x_{(i − 1)},x_{(i + 1)}] lies outside the boundary of P. (see also concave polygon)