What is the difference between "which" and "that? What should I use in the sentence? [duplicate]

Solution 1:

The Guardian style guide offers an opinion. Since it's online and free, I'll quote it in full:

that or which?

The traditional definition is that “that” defines and “which” informs (gives extra information), as in:
“This is the house that Jack built; but this house, which John built, is falling down.”
“The Guardian, which I read every day, is the paper that I admire above all others.”
“I am very proud of the sunflowers that I grew from seed” (some of them); “I am very proud of the sunflowers, which I grew from seed” (all of them).
Note that in such examples the sentence remains grammatical without “that” (“this is the house Jack built,” “The Guardian is the paper I admire above all others,” “I am very proud of the sunflowers I grew”) but not without “which” (“this house, John built, is falling down”).

A word about relative clauses: restrictive relative clauses (also known as defining, best thought of as giving essential information by narrowing it down) are not enclosed by commas, whereas non-restrictive relative clauses (non-defining, giving non-essential information) are.

In the three examples, “which John built”, “which I read every day” and “which I grew from seed” are all non-restrictive. They give extra information, they are preceded by a comma, and they use “which” rather than “that”. If you try them with “that” they sound odd (“the Guardian, that I read every day”). It’s not the same the other way round: although “that” is more common in restrictive clauses, you can use “which”: “the Guardian is the paper which I read every day”.

A formula that may help simplify things:
Restrictive clauses – “that” (desirable), no comma (essential).
Non-restrictive clauses – “which”, comma (both essential).
So a BBC radio interviewer who asked the question “should advertising, which targets children, be banned?” was suggesting that all advertising targets children. She meant “should advertising that targets children be banned?”

So which you use actually depends on what your sentence is intended to mean.

They describe different methods that their company usually practices.

The company uses certain methods and these are described. Usually here, "different methods" would be qualified by a determiner like some or the.

They describe different methods, which their company usually practices.

They describe some methods, and it's conincidental that the company uses them.

Solution 2:

From Webster's Dictionary of English Usage p894-895:

"That," "which" introducing restrictive and nonrestrictive clauses.

That is our oldest relative pronoun. According to McKnight 1928 that was prevalent in early Middle English, which began to be used as a relative pronoun in the 14th century, and who and whom in the 15th. That was used not only to introduce restrictive clauses, but also nonrestrictive ones:

Fleance his son, that keeps him company - Shakespeare, Macbeth, 1606

By the early 17th century, which and that were being used pretty much interchangeably. Evans 1957 quotes this passage from the Authorized (King James) Version (1611) of the Bible:

Render therefore unto Caesar the things which are Caesar's; and unto God the things that are God's.

During the later 17th century, Evans tells us, that fell into disuse, at least in literary English. It went into such an eclipse that its reappearance in the early 18th century was noticed and satirized by Joseph Addison in The Spectator (30 May 1711) in a piece entitled "Humble Petition of Who and Which against the upstart Jack Sprat That." That had returned, and although it could still be used to introduce a nonrestrictive clause,

Age, that lessens the enjoyment of life, increases our desire of living —Oliver Goldsmith (quoted in Lurie 1927)

this function was much reduced. Its nonrestrictive function continued to diminish, and although it was still so used in 19th-century literature, by the early 20th century such use seemed anomalous enough that Fowler 1907 singled out these examples (and others) for censure:

And with my own little stock of money besides, that Mrs. Hoggarty's card-parties had lessened by a good five-and-twenty shillings, I calculated . . . — Thackeray

How to keep the proper balance between these two testy old wranglers, that rarely pull the right way together, is as much . . . —George Meredith

As to dictionaries of the present day, that swell every few years by the thousand items, the presence of a word in one of them shows merely . . . —Richard Grant White

The brothers Fowler may have been prompted to find nonrestrictive that anomalous by the opinions some grammarians expressed around the turn of the century. Hall 1917 cited several of these, who seem to have felt that nonrestrictive that had always been rare or had become so lately. Hall thought the grammarians had not looked very hard at English literature; he did, and listed some 115 authors who used nonrestrictive that (in some 1100 passages). About half of his authors are from the 19th century. Hall made one important point that no one else seems to: poets are the heaviest users of nonrestrictive that. The reason is fairly obvious: that flourishes in unstressed positions where which will not fit comfortably. Grammarians and usage commentators tend to look at prose. It may well be that the historical tendency of that to be less often used in introducing nonrestrictive clauses has always been more marked in prose than in poetry and speech. At any rate, Virginia McDavid in American Speech (Spring-Summer 1977) reports a study showing that to introduce only restrictive clauses in mid-20th-century edited prose.

The finding of 1977 should satisfy you if you are writing prose. No one seems to have considered poetry since Hall in 1917. The nonrestrictive that is not entirely dead, however; Evans 1957 hinted at its continuing use, but his two unidentified examples may be from poetry (or even older prose). We do find the use occasionally in represented speech and in speechlike prose (as, for instance, a chatty letter not intended for publication):

"I mean little Sid Mercer, that rides for me. He's the duke of them all when he lays off the liquor...." — Ring Lardner, The Big Town, 1921

" . . . Take while I'm in an offering mood. I'm not the Red Cross that you can call at any emergency." — Mordecai Richler, The Apprenticeship of buddy Kravitz, 1959

When I was in the hospital even the nurses' aides that didn't have sense enough to do anything but empty the ice-water were full of that chatter —Flannery O'Connor, letter, April 1956

And in January 1969 Theodore Bernstein in Winners & Sinners took time to censure two instances of nonrestrictive that that had appeared in the New York Times earlier in the month. He said that he had not had to mention such a use for years. The evidence seems to indicate, however, that nonrestrictive that is still natural to some people, even though it is not used in edited prose. The examples Fowler 1907 gives of nonrestrictive that show that commentators and grammarians were then well aware of its diminishing range. And if that was being confined to introducing restrictive clauses, might it not be useful (as well as symmetrical) to confine which to nonrestrictive clauses? The Fowler brothers thought so, as perhaps some of their predecessors had: Ayres 1881 corrected a which to a that in a restrictive clause, observing that such is the practice of "our most idiomatic writers." Bierce 1909 made a similar correction. Fowler 1926 put the proposition succinctly:

. . . if writers would agree to regard that as the defining relative pronoun, & which as the non-defining, there would be much gain both in lucidity & in ease. Some there are who follow this principle now; but it would be idle to pretend that it is the practice either of most or of the best writers.

Evans 1957 responds: "What is not the practice of most, or of the best, is not part of our common language."

Evans's commonsensical observation did not occur to, or did not impress, most subsequent usage writers, who remember only Fowler's first sentence. The general recommendation of the majority is to follow Fowler's wish, although many of them hedge the recommendation round with exceptions, caveats, and appeals to euphony or formality.

But which is as firmly entrenched in its restrictive function as in its nonrestrictive one. Joseph M. Williams in College Composition and Communications (May 1981) points out that even some of those who recommend using that instead of which in restrictive function use which themselves unawares. For instance, Jacques Barzun, in Simple & Direct (1975), says this in the middle of one page:

In conclusion, I recommend using that with defining clauses, except when stylistic reasons interpose. [The sylistic reasons discussed refer to a succession of thats.]

And this to open the first paragraph on the next page:

Next is a typical situation which a practiced writer corrects "for style" virtually by reflex action: . . .

Williams also cites the discussion of which and that from Strunk & White 1959, which recommends "which-hunting," and then quotes White's own usage:

. . . the premature expiration of a pig is, I soon discovered, a departure which the community marks solemnly on its calendar —E. B. White, "Death of a Pig"

If the discussions in many of the handbooks are complex and burdened with exceptions, the facts of usage are quite simple. Virginia McDavid's 1977 study shows that about 75 percent of the instances of which in edited prose introduce restrictive clauses; about 25 percent, nonrestrictive ones.

We conclude that at the end of the 20th century, the usage of which and that — at least in prose — has pretty much settled down. You can use either which or that to introduce a restrictive clause — the grounds for your choice should be stylistic — and which to introduce a nonrestrictive clause. A number of commentators raise the additional question of the relative formality of that and which. If you read many of them, you will find their observations contradictory. Formality does not seem to be much of a consideration in the choice of that or which.