Acromioclavicular Joint

The acromioclavicular joint, or AC joint, is the synovial joint between the acromion (part of the scapula that forms the highest point of the shoulder) and the clavicle. 

The acromioclavicular joint functions as a pivot point (although technically it is a gliding synovial joint), increasing the range of rotation of the scapula on the thorax and resulting in a greater degree of arm rotation.