The difference between that and which has evolved over the years. At one time they were interchangeable. Then grammarians got hold of them around the end of the nineteenth century and thought it would be a good idea to subject them to certain rules. Later still, eminent writers baulked at being so restricted, leading us to realise their modern usage: only which can introduce a non-restrictive clause, otherwise the two are interchangeable (both can be used in restrictive clauses).
So, what does this mean?
Restrictive and non-restrictive
A restrictive clause is otherwise termed, “a defining clause”. It is essential to the meaning of the sentence. Remove the clause and it won’t make sense. In this case you will normally use that, but perhaps which in some instances. Incidentally, restrictive clauses are also introduced by relative pronouns (aside from that): who, whom and whose.
A non-restrictive clause is otherwise termed, a “non-defining clause”. It is non-essential to the meaning of the sentence and can be omitted. In this case you will always use which.
One way to check the correct usage is to remove a which clause from a sentence. If the meaning is lost then you should have been using that instead of which. Put the clause back and change it to that. If you remove a that clause from a sentence and it still makes sense, then you should have been using a which clause. In this case replace that with which and you must then precede it with a comma.
For example, consider this sentence:
- “The basket, which is made from raffia, has bread in it.”
If you remove the clause “which is made from raffia” the meaning of the sentence is not lost. This confirms it is a non-restrictive clause and it is the correct to use which.
Only which can be used to introduce non-restrictive clauses. They must have a comma before and after them (or only before them if they come at the end of a sentence). The commas indicate the removable part, because a non-restrictive clause can be left out without changing the meaning of the sentence:
- “Benches, which are found in many school assembly halls, are uncomfortable to sit on.”
- “Benches are uncomfortable to sit on.” [clause omitted]
- “The pupils have been complaining about the textbook, which is hard to follow.”
- “The pupils have been complaining about the textbook.” [clause omitted]
Only which before a comma
By the same token, placing a comma before that is incorrect.
- “This computer, that I have never liked, is very slow.” [incorrect use of that]
- “This computer, which I never liked, is very slow.” [correct use of which]
- “The oak desk, that my grandfather gave me, is very old.” [incorrect use of a comma]
- “The oak desk that my grandfather gave me is very old.” [correct use of that]
There are times—rare to be sure—when there should be a comma in a sentence before that, for example, where it is used parenthetically:
- “I don’t know anything about the murder, that is, not until I read it in the paper.”
As with grammar in general one must avoid slavish adherence to rules and rather apply common sense. This sentence rigidly follows the rules, but it is ambiguous:
- “It emerged that Susan made the complaint, which surprised everybody.”
This could mean either that the complaint was surprising or that it was surprising that Susan made it. The ambiguity can be avoided by using a paraphrase instead:
- “It emerged that Susan made the complaint, a revelation that surprised everybody.”
Which is flexible
As we have said which seems to have a bit of flexibility in modern grammar. It can be used in restrictive clauses too, but in certain situations:
- “Who took the documents that I brought in yesterday?” [stylistically preferred!]
- “Who took the documents, which I brought in yesterday?” [accepted usage]
- “How can you govern a country which has 246 varieties of cheese?” [French President Charles De Gaulle, presumably it was originally in French!]
Which is preferred to introduce a restrictive relative clause when the preceding phrase itself contains a that:
- “I can only give you that which I don’t need.”
- “We want to assign only that book which will be most helpful.”
That should be used to introduce a restrictive clause. As we have said, a restrictive clause is necessary to the meaning of the sentence. In this example:
- “I want the basket that has bread in it.”
If you removed the phrase “that has bread in it” the meaning of the sentence is lost.
That has many guises
Whereas which has only one persona, that has many guises. It is used as a determiner, a demonstrative pronoun, a relative pronoun, a conjunction, an adjective, and an adverb.
That as a determiner
This is probably the most common use of the word that, to point to something or someone.
A determiner is a word that is followed by a noun:
- “Give me that rain jacket.”
- “Can you let me have that typewriter you said you were no longer using.”
In each case the determiner that is followed by a noun: rain jacket, typewriter.
That as a demonstrative pronoun
A demonstrative pronoun is a pronoun that points to something. That is a member of this little group, along with this, these and those. They always identify nouns:
- “That looks like the car I used to drive.”
- “Is that yours?”
That as a relative pronoun
That as a relative pronoun points to a person, a thing, or a group. It defines the subject being talked about or understood. A relative pronoun relates to and modifies a noun that has already been mentioned. Aside from that the most common relative pronouns are: what, which, who, whoever, whom, whomever, and whose.
Who and whom refer to a person; which refers to animals or a things; what refers to inanimate things; while that refers to a person, animal or thing:
- “The trousers that I bought last week are already in the wash.”
- “John visited the pub that had recently been in the news.”
That as a relative pronoun can be used to join sentences:
- “He came to collect his brief-case.” [first sentence]
- “His brief-case had been left behind in his study.” [second sentence]
- “He came to collect his brief-case that he had left behind in his study.” [combined sentence]
In its role as a relative pronoun, that can sometimes be substituted for who and which, depending on the context:
- “Geraldine bought a pair of gloves, which the man was selling at the market.”
- “Geraldine bought a pair of gloves that the man was selling at the market.”
- “Sheila invited the girl who was free today.”
- “Sheila invited the girl that was free today.”
That as a conjunction
That can be used as a conjunction. In this case it joins two clauses.
- “I did not know. She was married.”
- “I did not know that she was married.”
- “It made such a noise. We had to cover our ears.”
- “It made such a noise that we had to cover our ears.”
That as an adjective
That can be used as an adjective, again pointing to something. Consider:
- “Do you remember that talk we had?”
- “Later that week I went to visit my mother again.”
- “That’s not what I said.”
- “Oh, that’s really terrible!”
That as an adverb
- “I came that close to crashing into the wall.”
- “I did not think that he would be so angry.”
That as an intensifier
That (again as an adverb) can also be used as an intensifier, playing a similar role to common intensifiers such as “very”, “extremely” or “terribly”.
- “Oh, no, not that!”
- “I did not think Emily was that good.”
Omission of that
That can sometimes be omitted without affecting the meaning:
- “OK, so I admit that I made a mistake.”
- “OK, so I admit I made a mistake.”
- “I do not believe that Celia was at home.”
- “I do not believe Celia was at home.”
- “The book that I was reading gave me nightmares.”
- “The book I was reading gave me nightmares.”
Omission of that in these cases has sometimes been described as incorrect, but common usage outvotes the pedantic grammarian! However, the omission is a matter of common sense. In this sentence:
- “John said that under no circumstances would he allow us to miss his meeting.”
- “John said under no circumstances would he allow us to miss his meeting.” [still makes sense]
However, in this sentence:
- “The manager says that eventually the housing supply will improve.”
- “The manager says eventually the housing supply will improve.” [ambiguous]
Omission of that here creates ambiguity because the adverb eventually could be construed as modifying either says or will improve.
Those is the plural of that.
- “I need to paint that door.”
- “I need to paint those doors.”
- “Did you enjoy that trip we made last year to Paris?”
- “Did you enjoy those trips we made last year to the mountains?”
You should usually prefer to use who and whom instead of which or that in reference to people (and animals with names, such as pets). Examples of who when referring to people:
- “The girl who threw the tennis ball is a nuisance.”
- “This is the man who is always causing trouble.”
Use of what and which
The distinction between the use of these two words depends on the antecedent in a sentence. The antecedent is a word to which a following word refers. In:
- “I will give this to Jeremy if I see him.”
“Jeremy” is the antecedent of “him”. In other words, him is the following word that refers back to the antecedent Jeremy.
Thus, which should be used when it is preceded by its antecedent:
- “Even so, he has not said he will withdraw, which is surprising.”
On the other hand, what should be used when the antecedent follows:
- “Even so, what is surprising, he has not said he will withdraw.”
That is used in a restrictive clause to point to someone or something. The clause is essential to the meaning of the sentence and cannot be omitted. No comma is required. In some cases the word that can be left out without affecting the meaning. In referring to persons, who or whom might be preferred.
Which is used in a non-restrictive clause. It always follows a comma. It is non-essential to the meaning of the sentence. Removal of the clause would not affect its meaning.
Which can also head up a restrictive clause, where it is used to refer to something (not someone).
- That clauses are restrictive (essential).
- Which clauses are non-restrictive (non-essential). And they need commas.
By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar
Last updated: Monday, 20th April 2020