The standard rule of grammar is that the usage of that vs. which depends upon whether the following clause is restrictive or non-restrictive.
"That" is used to indicate a specific object, item, person, condition, etc., while "which" is used to add information to objects, items, people, situations, etc. Because "which" indicates a non-restrictive (optional) clause, it is usually set off by commas before "which" and at the end of the clause.
Rule for using That instead of Which
The general rule requires that you use that only to introduce a restrictive (or defining) relative clause, which identifies the person or thing being talked about. For example,
The building that I was telling you about is just down the road.
In this sentence, the phrase that I was telling you about specifically identifies the object in the previous phrase (building) and is a restrictive clause. In this use, that should never be preceded by a comma because the word is an integral (non-optional) part of the description.
Similar examples include:
- My books that have red covers are new.
- The classes that are held every Monday start at 9:00 AM.
Note that the subject of the restrictive clause can change "that" to "who", "when" and "where" for correct usage. Use who for a person, when for a time period and where as a substitute for "that place". For example:
- The man who shot Lincoln jumped onto the theater's stage.
- Remember the time when I fell off the ladder?
- Billy went to where they sell turkey eggs.
Rule for using Which instead of That
On the other hand, use which with non-restrictive (or non-defining) clauses. These are clauses providing additional information about something that has already been identified in the context. In this use, which is always preceded by a comma and a comma is placed after the restrictive clause ends (if the sentence continues). For example,
- My new books, which have black covers, are on the desk.
- The padded chairs, which are on the second floor, have to be replaced.
When the clause is at the end of the sentence, only one comma is used, before which:
The students in Chemistry 101 have been complaining about the textbook, which is hard to follow.
In this case, the clause which is hard to follow is descriptive, not restrictive i.e. it does not specify which text is being complained about (although it can be easily inferred). For these cases, which sounds more natural than that.
Some grammarians extend the rule and insist on that being used only in restrictive clauses, while which should be used only in nonrestrictive clauses. For example:
Wrong, according to strict grammarians: I need a book which will tell me all about city gardening.
Correct usage: I need a book that will tell me all about city gardening.
The use of which with restrictive clauses is fairly common, even in edited prose. However, the American Psychological Association (APA), in its 6th edition Manual, recommends adhering to the rule and use that for all restrictive clauses.
Which can be especially useful where two or more relative clauses are joined by and or or. For example,
Politics is an environment in which ordinary people may succumb to greed and which many have found reason to hate.
You may also want to use which to introduce a restrictive clause when the preceding phrase contains a that or a those. For example,
- We want to fund only those initiatives which will increase revenues and customer satisfaction.
- Mrs. O'Reilly likes that new Italian restaurant which is located downtown.
Using That and Which together
The words that and which can also be used together. In doing so, that serves to emphasize the descriptive clause introduced by which. For example,
Reality is that which, when you stop believing in it, doesn't go away.
GMAT Test Questions
The Graduate Management Admission test (GMAT) has a Sentence Correction section. To improve your chances at properly selecting the right answer between that and which, use this technique:
- Does eliminating the clause change the meaning of the sentence? Use THAT.
- If the meaning doesn't change, does the clause describe something more about the subject? Use WHICH.
- The Grand Canyon is a U.S. National Park that/which is in Arizona. [Use which]
- My brother doesn't like sports that/which are played indoors. [Use that]