Merriam-Webster - That vs. Which lyrics

Hi! Welcome to Ask the Editor. I'm Kory Stamper, an Associate Editor at Merriam-Webster. While corresponding with someone a while ago, I wrote: "The book which addresses this topic is a usage dictionary." My correspondent took great joy in pointing out that I was wrong for using "which" instead of "that." Was I? There is a rule about the use of "that" and "which." It's this: "that" is used to introduce a restrictive clause, that is, a clause with the meaning that's essential to the sentence; "which" is used to introduce a nonrestrictive clause, a clause with the meaning that's not essential. Take the sentence "The book that addresses this topic is a usage dictionary." The restrictive clause "that addresses this topic" is essential. Without it, we don't know what book is being talked about. The sentence "The book, which is blue, is a usage dictionary" has the nonrestrictive clause "which is blue." Although it's nice to know the color of the book, we don't actually need that information. So why would I use "which" in a restrictive clause? Because English speakers have been doing that for centuries. Long ago, "that" and "which" were both used to introduce both kinds of clauses. "That" fell out of literary use during the 17th century but resurfaced a few decades later, at which point some grammarians decided it would be tidy to make "that" the king of restrictive clauses and "which" the king of nonrestrictive ones. But, as is often the case, many English speakers didn't get that memo. "That" settled comfortably into its restrictive clauses role, but "which" was used—and continues to be used—in both restrictive and nonrestrictive clauses. Should you care? Well, you should be aware that some people take the that/which rule very seriously. But if you prefer the restrictive "which," you can use it—because you know the history which precedes it. For more episodes of Ask the Editor, visit merriam-webster.com.