The movements occurring at joints with fully congruent articular surfaces, such as ball-and-socket joints and hinge joints, can be roughly infered from the geometrical forms of their articular surfaces. In these cases pure sliding occurs between the moving surfaces. But this is no longer true for incongruent joints, and in such joints the ligaments play an important role in determining the frequently complex nature of the movements these joints produce : with combinations of sliding and rolling between their moving surfaces. Finally, the instantaneous velocity of these moving surfaces is also strongly influenced by the arrangement of the ligaments.