Mathina Calliope, at Jane Friedman’s site, does a terrific job of explaining essential/restrictive versus nonessential/nonrestrictive commas, with lot of examples. I know from many observations that writers struggle with this distinction. It’s one of what I argue are “the only comma rules you’ll ever need,” but it’s the hardest to explain in a short space. So head over and check out her discussion.
Just Can't Help Writing 
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