Face is the element makes the mesh visible. In Unity, the process of rendering a face is broken down to 2 steps: Render front face and back face. So how can we determine which side is front face and which side is back? To form a triangle, you have to pick 3 vertices into a group, the order matter! When looking down to the mesh from the camera, front face is the side which its vertices order goes clockwise, and back face is the side which its vertices order goes counter-clockwise. However, it doesn’t matter which vertex you start with.