I take a rather strange position (probably to most on the forum), when it comes to the pectoral muscle, but it is based on how the muscle (s) are designed and some books I have read on the subject.
The biggest muscle group of the chest is the pectoralis Major, and it has an upper clavicle and middle sternocostal portion. There is a lower portion, which for some can attach to the top portion of the connective tissue of the abdominal muscle (s).
It is commoningly accepted that if you do incline presses (which will bring in more shoulder involvement, because shoulders are higher, and but not much talk on: Hand Position) will work (primarily) the upper portion of chest, and if one does decline presses (shoulders lower, and shoulders are less involved) one would work the lower chest (which I tend to regress...a tad on).
And, believe it or not, there is a lesson here, when one is having shoulder problems.
A study, I believe through the Journal of Strength and Conditioning Research (1995, I believe), tested the above common beliefs. In this study, putting the hands in a "more narrow position" (versus the traditional wide grip) on the incline press seemed to activate/improve the upper chest muscle recruitment,---and the decline press didn't activate the lower chest anymore than the flat bench press.
For the average gym training person, Incline and flat bench presses will suffice. Notwithstanding using Dumbbell variations to activate different stabilizers.
If you look at the they way the chest muscle extends, they primarily extend from (or fan out if you will) from the sternum, which means the angle goes one way. For example, the Dumbbell fly. When done properly, it follows the line of the muscle fibers.
A lot of persons believe that dips work the lower chest (this is not specific-position-accurate), a lot of stress/strain can be localized at the sternum and in cases pretty heavily with the new to weight training, but is an excellent "overall" chest/tri developer.
In speaking in terms of just free weight training: If you concentrate on Flat and Incline presses, and where appropriate add in some DB variations, you will be fine.
Best wishes,
Chillen
