This is a rule that can be applied to a lot of things, including composition and framing. Here I want to talk of this ‘rule’ (as any rule, it has exceptions!) for faces.
When you look at a face without any angle, straight at you, you can divide the face in three equal (more or less) parts.
It’s a very useful tip when it comes to drawing, as some people tend to place the eyes too high or too low.
Begin by drawing the oval of the face, then trace the mid line. Divide in three equal parts: under the nose and on the eyebrows. Don’t forget to add volume for the hair, as the top line is the skull!