Shoulder
The shoulder is composed of several bony structures, the scapula, humerus, and clavicle, which combine to create both the glenohumeral joint and the acromioclavicular joint. Four short muscles originate on the scapula and pass around the shoulder, where their tendons fuse together to form the rotator cuff. As the only joint in the body that can rotate a full 360 degrees, the shoulder is less stable than other joints, and is dislocated more easily.

Elbow
The elbow is a joint connecting the bone of the upper arm with the two bones of the lower arm. This joint contains three smaller joints, the humerus-ulna joint, the humerus-radius joint, and the radius ulna joint. The unique positioning and interaction of these bones allows both rotation and hinge action within the elbow. Primary stability is provided by the ulnar collateral ligament on the inner side of the elbow.