How do pectorals work?

The Pectoralis Major muscle attaches form the sternum, ribcage, and clavicle to the humerus. They share the function of Adducting the arm (pulling it down and in towards the body) when working with certain muscles of the back (like the latismuss dorsi). When working alone they pull the arm forward (from pointing out sideways to pointing forwards) the position of the "flexed" shoulder.