In MotionBuilder you can use two types of skeletons: bipeds or quadrupeds.

A bided skeleton has two limbs that touch the ground. For example, a human biped model displays in the following figure, A. In B, the skeleton of the same model displays with its mesh hidden.

A. Biped model B. Skeleton only

A quadruped skeleton has four limbs that touch the ground.

Quadruped skeleton