I was watching Tyler's Ultimate the other day while he was making Pernil and he used the term Boston Butt and Picnic Shoulder interchangeably on the show. I was confused.
The shoulder includes both the picnic and the butt.
The whole pork shoulder is sold just as it says, whole. The scapula or shoulder blade of the hog is severed from the main carcass, and everything from that point down to the last joint before the foot is sold as whole shoulder. If, however, you were to severe the meat once more in the middle of the entire leg portion, at the elbow, you would be left with two separate cuts. The 'top' cut, or that which goes to the scapula is known as the Butt Roast, or Boston Butt, and the remaining 'lower' cut is know as the picnic shoulder.