less than 180 degrees.
*Every line segment between two points in the interior, or between two points on the boundary but not on the same edge, is strictly interior to the polygon (except at its endpoints if they are on the edges).
*For each edge, the interior points and the boundary points not contained in the edge are on the same side of the line that the edge defines.
*The angle at each vertex contains all other vertices in its interior (except the given vertex and the two adjacent vertices).

Every non-degenerate triangle