Knee : Hinge joint :: Shoulder : ? Complete the analogy.

The relation is a joint of the body : the kind of joint it is, named after the way the bones move upon each other. The knee swings to and fro in one plane only, like a door upon its hinge. The shoulder turns almost every way because the rounded head of the arm bone sits in a shallow cup, which is a ball and socket. Each distractor names a real kind of joint found elsewhere — the pivot lets the head turn upon the neck, a fixed joint locks the plates of the skull so that nothing moves, and a gliding joint lets the small wrist bones slide over one another.