By using ungulates to illustrate the idea, I have attempted to explain the observation that the mammalian jaw joint almost always lies above the level of the upper tooth row. I describe how the same set of bilaterally symmetrical muscles, with relatively fixed directions of pull and without any forces in the wrong direction, are not only able to bring the teeth together but can also move the lower jaw in two different directions (i.e., medially from either side). Aspects of this explanation may be generally applicable since the ungulates and the mammals studied so far are similar in that they exhibit a medially directed power stroke with bilateral activity of corresponding masticatory muscles during trituration of food as well as jaw joints that lie above the upper tooth row.