When you have a triangle in 3-D space, it has two sides, the front side and the back side. Since most graphics images are composed of triangles, we needed a way to distinguish the front of the triangle from the back.The convention has been established that you are addressing the front side of the triangle when you state its vertices in a counter clockwise direction. You can see that these same points would be in clockwise order if you look at the same triangle from behind.