The shoulder is the area around the shoulder joint (glenohumeral joint) — the joint where the upper arm joins the shoulder blade. The shoulder blade, in turn, has a joint with the clavicle or collarbone that connects to the sternum at the center of the chest. This construction means that the shoulder joint is not fixed to the ribcage: it has freedom to move up and down, and also back and forward, but always keeps a fixed distance from the sternum.
In figure drawing, it is useful to know and remember that children's shoulders are more narrow than those of adults, in relation to the size of the head. Also, shoulders tend to be wider in men than in women.