Did you know that either the lower edge of the upper lip, or the upper (red) edge of the same lip very often has the same angle as the septum? For most people, there is a correspondence.

I learned about this some 20-30 years ago, and have spent many hours on the subway staring at people, confirming that these parallel angles are highly common -- I'd call it "normal", in fact. Unfortunately, I no longer remember where I learned it, but I was studying Leonardo’s drawings at that time, so maybe it came from there.

This correspondence changes over the years, as the cartilage of the nose continues to grow all our lives (as do our ears). But, as gravity takes its toll on the upper lip too, it doesn't change as much as one might think -- not in all individuals.

This correspondence of angle is easiest seen in a neutral, fairly relaxed face. Even a hint of a smile offsets it.

In the picture, a to d shows the parallel lines. In e, the black arrows point to the fact that  if the nostril has a high curve, so does the upper edge of the upper lip. If the nostrils are flat, then the mouth has a very flattened M-shape (or Cupid's Bow, as we say in Sweden.)

The man in f belongs to a very common variation of the 'rule' -- his septum corresponds to the angle of the lower lip, when the mouth is relaxed and slightly open. The more pronounced the beak is, the steeper the angle of the lower lip. Yasser Arafat is an excellent example. But when the mouth is closed, it looks straight, or slanted as the first examples.

Look at people around you, when they are neutral or relaxed, and see if this is indeed true. Eventually you'll detect it in faces seen from straight on, too.