A principal vertex 
of a simple polygon 
is called a mouth if the diagonal ![[x_(i-1),x_(i+1)]](/images/equations/Mouth/Inline3.svg)
is an extremal diagonal
(i.e., the interior of ![[x_(i-1),x_(i+1)]](/images/equations/Mouth/Inline4.svg)
lies in the exterior of 
).
Mouth
See also
Anthropomorphic Polygon, Ear, One-Mouth TheoremExplore with Wolfram|Alpha
References
Toussaint, G. "Anthropomorphic Polygons." Amer. Math. Monthly 122, 31-35, 1991.Referenced on Wolfram|Alpha
MouthCite this as:
Weisstein, Eric W. "Mouth." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Mouth.html