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a picnic is the shoulder and arm. It usually comes with a layer of skin. As solar said a whole shoulder includes the butt
I got this off the web
The first, and probably most common, is Boston butt. Boston butt comes from high on the hog, above the shoulder blade, and has lots of juicy, marbled fat. This is probably what most people buy when they purchase pork shoulder in a grocery store. It's a versatile piece of meat that you can just as easily roast or braise. For home cooks, it's also very inexpensive.
Below the butt is the pork shoulder. This cut includes most of the hog's front leg quarter. Because the leg muscles work a lot more than the back, the meat is a little tougher here than the butt, thus requiring a bit more time to coax out tenderness.
I've talked with a couple of butchers and they said that the term Boston Butt is a regional name that's not used in my area but when I asked them about the pork shoulders they were selling and how they compare to Boston Butt they said they're the same thing. But you're (boykjo) saying the pork shoulder is below the butt. I thought the picnic was below the butt.
With a little study, you will readily recognize them.
In my area the butt is usually called pork butt or a Boston butt or shoulder butt roast. The different terminology would be confusing, but you will soon be able to identify them by sight.