The clavicle, which is also known as the "collarbone," serves numerous functions, one of which is as a strut to dangle the arm out of the thoracic cage's width, according to Boundless. The clavicle is wider than the ribcage, so the arm is connected to its end, clearing the thorax.
The scapula, or "shoulder blade," also serves to keep the arm from colliding with the ribcage, according to Boundless. The scapula pushes against the ribcage to provide balance and leverage whenever the arm is lifted, pushed forward or pulled backward. It is the combination of three pieces (arm, clavicle and shoulder blade).