Commas are called to duty to surround non-restrictive clauses. A non-restrictive clause adds non-essential information to the sentence, i.e. the meaning of the sentence remains clear if the non-restictive clause is deleted.
The April issue of the magazine, which was displayed by the cash register, contained several feature articles.
Commas are non-essential and may take the day off if a restrictive clause supplies relevant information to a sentence.
The book which has a red sticker on the back belongs on the bottom shelf.
After all, we wouldn’t want to put just any book on the bottom shelf.
The Comma Archives 