The American "pork shoulder butt" is the upper, or butt, end of the pig's foreleg, including only the muscle and bone of the shoulder joint and upper back.

The pig's actual butt, or bum, is the ham.

(I also translate from French.)

I'm with Steve Shaw on the issue of pork lusciousness -- the shoulder wins by miles. Loin and leg roasts benefit hugely from smoking and curing, which is why ham is so much easier to find than a fresh leg roast of pork. It also explains all the variant forms of bacon, of which my favourite is Canadian-style peameal ...

Hmmmm. Let's talk about bacon.