The bony anatomy of the shoulder begins with the upper arm (humerus), the collar bone (clavicle) and the shoulder blade (scapula). The main shoulder joint is a ball-and-socket joint formed between the humerus and the scapula (darker blue arrow). This ball-and socket construction gives the shoulder joint great mobility. The acromio-clavicular joint ("A-C") joint is formed where the collar bone and shoulder blade meet (light blue arrow).