In a new episode of her humorously informative grammar series, the very knowledgeable Mary Norris aka “The Comma Queen” explains the difference between relative pronouns such as “that” and “which” and how the New Yorker puts them to use.
Today we’re going to go for the jugular and talk about ‘that’ and ‘which’. This is something that people argue about all the time and I’m going to explain how we do it at the New Yorker. ..if the phrase is essential to the meaning of the sentence, it’s attached to the main clause with ‘that’ and no comma. If the clause is not essential to the meaning of the sentence use a comma and the word ‘which’ to attach it.