The two relative pronouns "that" and "which" are often used interchangeably. However, a grammar rule determines when a writer should use one or the other.
Here is the rule: use "that" ONLY with essential/restrictive clauses; "which" is used mostly with nonessential/nonrestrictive clauses (but can be used with restrictive clauses). So, what is the difference between a restrictive and non-restrictive clause? The sentences below illustrate the distinction:
Restrictive Clause: The study revealed a formal policy THAT provides a way to report HR violations anonymously is missing from many small agencies.
Essential or restrictive clauses are necessary parts of a core sentence.
Conversely, nonrestrictive clauses (also known as nonessential clauses) are not needed elements of the core sentence; therefore, commas are placed around them to note they are not part of the main sentence. See the sentence below for an example:
Nonrestrictive Clause: Several researchers theorize about the lack of accountability at the highest levels of government agencies, WHICH leads to the gaps in oversight.
Notice a comma precedes the pronoun "which" in this sentence, indicating what comes after the comma is not a necessary part of the sentence. If a writer wants to make that information a part of the core sentence, then the writer should delete the comma and change "which" to "that."
When determining whether to use "that" or "which," you must first identify if the words after the pronoun are part of the core sentence. If they are, you most likely should use "that." If the words are not a part of the core sentence, you definitely should not use "that."
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