What type of joint is the shoulder?

The shoulder is a ball-and-socket joint. More precisely, the glenohumeral joint joins the rounded head of the humerus to the shallow glenoid cavity of the scapula, or shoulder blade.

This design allows the arm to move through a wide range: forward and backward, away from and toward the body, and in rotation. The shoulder is the most mobile major joint in the human body, but that mobility comes with a trade-off. Its shallow socket makes it less inherently stable than the deeper hip socket, so shoulder dislocations are relatively common.

A ring of tough fibrocartilage called the glenoid labrum deepens the socket, while the rotator cuff muscles and tendons help hold the humeral head in place. The shoulder region also includes the acromioclavicular and sternoclavicular joints, which help the shoulder blade and arm move, but the joint usually meant by this question is the glenohumeral joint.

A common mix-up is calling the shoulder a hinge joint because the arm can bend and straighten. It does that motion, but its ball-and-socket structure also permits circular movement and rotation.