A polygon is said to be convex if no line that contains a side of the polygon contains points in the interior of the polygon. Two convex polygons are presented in the illustration below with the lines that include the sides of each polygon.

All triangles are convex polygons. A polygon with four or more sides is convex if all internal angles are less than or equal to $180^{\circ }.$ If all internal angles are strictly less than $180^{\circ },$ the polygon is said to be strictly convex. See also concave polygon.