There are a few exceptions to this rule, but it will serve you well to follow it and to know when to break it. Strait lines drawn on either side of the centerline that connect all of the major features of the face, edges and orifices, are all parallel.
This is easily one of the most common mistakes that unseasoned artists make, and a few seasoned ones too, including yours truly. When it is not right, your eye notices it immediately and the drawing or painting looks off. I will give additional tips on the proportions of the face, that are a bit more complex, but this one is an easy rule to follow.
So before you start writing me back about how I am wrong and you are looking at a photo that doesn’t follow the rule, here are a few exceptions.
Exception 1. If someone’s face is so asymmetrical that the features don’t line up in an even structure.
Exception 2. The person is making an expression or pushing on one side of their face that pulls at the softer features and changes the balance of those landmarks.
Exception 3. You are so close, or the photo was taken so close that those lines are in perspective and converge at a vanishing point.
That’s it. Otherwise parallel, parallel, parallel.