A chimpanzee skull: the jaws stick out in front this causes a prognathic face.
A human skull: the normal human face is basically vertical this causes an orthognathic face.
Prognathism describes when part or all of the face sticks out more than normal. Humans have orthognathic faces. That means the faces is more or less vertical. Most other primates have prognathic faces: their jaws stick out in front of the top part of the head.
- Nasal prognathism: is when the portion of the face around and including the nose sticks out. Also called upper facial projection.
- Alveolar prognathism: is when the alveolar (dental) part of the upper jaw (maxilla) sticks out. Also called mid-facial projection. Often causes what is called an "overbite".
- Mandibular prognathism: is when the mandible (lower jaw) sticks out. Often called an "underbite".