That & Which

Many have thought it would be good were that always to be used to introduce only restrictive clause modifiers (The big dog that is barking is a nuisance) and which, only nonrestrictive ones (The big dog, which is barking, is mine). This neat dichotomy has been much recommended, and some conservative watchdogs of our Edited English do follow it pretty generally. But—especially in Conversational or Informal contexts—most of us use which almost interchangeably with that in restrictive modifiers and rarely but sometimes use that to introduce nonrestrictive modifiers. Then too we often omit any relative at all, as in the car I want to own, rather than the car that I want to own or the car, which I keep in the garage. (See OMITTED RELATIVE.) Best advice: use that or which or nothing, depending on what your ear tells you. Then, when writing for certain publications, know that you may have to replace a good many whiches with thats, and perhaps a that or two with a which, to conform to the “rule” almost no one follows perfectly in other than Edited English and few can follow perfectly even there.