The line-of-action of the resultant vector of jaw muscle force is inclined posteriorly in most orders of mammals. This inclination varies directly with the height of the jaw joint above the level of the upper tooth row. A very high joint is paired with a near vertical line-of-action. A low joint is associated with a vector with a strong posterior inclination.

A few orders of mammals contain species in which the rostral region of the jaw has elongated over evolutionary time. This elongation did not change the 3:7 ratio of the jaw regions. Concomitant changes in the masses of the jaw muscles shifted the inclination of the resultant muscle force vector anteriorly. This changed the moment arms of the vector, and the teeth, thus maintaining the ratio. Other than jaw lengthening, major changes in the jaws did not take place.

A study of jaw muscle size and third molar location provides an estimate of the anteroposterior location of the resultant vector of jaw muscle force. This estimate agrees with the analyses described above as well as with estimates derived from dissections of the muscles.