Closure

Closure occurs with an incomplete space or a item that is not completely closed off. Yet, your eyes can still finish off the shape's or items outline. Below are two examples of closure. They both depict the weapon "a mace". The way a mace is shaped, it's a sphere with cone shaped spikes on it. So with that "3D element" only the spikes are depicted. Your brain automatically closes the sphere. Now, in the grey value piece, you also more of a 3D element before the lighting aspects bouncing off the cone shapes. Making them feel more 3D to bring it all together.