[HOpenGL] Convex polygons

I'm reading the red book and it says opengl cannot render convex polygons. Yet, when I compile this code: renderPrimitive Polygon $ do vertex $ Vertex2 0 (0::GLfloat) vertex $ Vertex2 0 (3::GLfloat) vertex $ Vertex2 4 (3::GLfloat) vertex $ Vertex2 3 (1.5::GLfloat) vertex $ Vertex2 4 (0::GLfloat) The convex polygon is displayed correctly. Does hopengl do something behind the scenes or I just "lucked" out that my opengl/hardware can handle this particular convex polygon case?