The GL_QUADS and GL_QUAD_STRIP flags do not have to be coplanar. OpenGL would probably just split them into triangles if the points were not coplanar. The polygon does not have to be coplanar eitherâ€¦but Iâ€™m not so sure about the polygon. I just remember the red book saying it has to be convex.

if you use a GL_POLYGON, than all vertices have to lie on a plane.
If you draw one GL_QUAD itÂ´s the same. However if you draw more than one quad (or a quadstrip), than they donÂ´t have to lie in one plane.
That means, if you draw 2 GL_QUADS, then the first 4 vertices have to be on one plane (to form the first quad), and the second 4 vertices have to be on one plane (to form the second quad). However the first quad does not have to lie on the plane of the second quad.