The pectoralis muscle originates on the sternum, upper 6 rib cartilages, and medial aspect of the clavicle, and inserts on the lateral lip of the greater tubercle of the humerus. Great, what good is that you ask. Well, knowing where the muscle originates and inserts helps you to better understand the movements the pectoralis muscle is responsible for. For practical reasons, you should know that the pectoralis adducts (brings towards mid-line) the upper arm and elbow (think of performing pec deck or fly exercises), forward flexes the upper arm (as in the bench press movements), as well as assists with internal rotation of the upper arm (as in cable crossovers).
There are 3 distinct regions within the muscle that will develop depending upon the exercises you perform. It is important that you train the upper, middle and lower regions of the pectoralis muscle, as well as begin with the exercise which focuses on the weakest area of your chest. For most people that's the upper chest. And for this reason, we begin our workout with the incline movement.