“That” versus “Which”
One of the most common errors I come across is when writers use “which” instead of “that.” The Chicago Manual of Style, 16th Edition, includes these words in its Glossary of Problematic Words and Phrases. Both words are relative pronouns in that they introduce a dependent (relative) clause and relate it to an independent clause (modifying a word, phrase or idea). The most commonly used relative pronouns are who/whom, whoever/whomever, whose, that, and which. (What, when, and where can function as relative pronouns in certain situations.)
There are two types of relative clauses: restrictive and non-restrictive. Relative pronouns that introduce restrictive clauses are not separated from the independent clause by a comma. The information they offer is essential to the meaning of the sentence. Example: The short story that you submitted was the winning entry. If you remove the relative clause (that you submitted) the sentence would not reflect its intended meaning. The relative clause, in this case, is restrictive; it provides necessary information. In the example, “that” is used restrictively to narrow a category or identify an item.
A relative clause is non-restrictive if it can be eliminated without changing the meaning of the sentence. Example: Your short story, which I read this morning, was the winning entry. Non-restrictive relative clauses are separated from the rest of the sentence by commas (or enclosed in parentheses or preceded by a dash). The use of “which” in the example provides additional information for an item already identified (your short story was identified as the winning entry). “Which” is used restrictively only when it follows a preposition.
“That” and “which” are not interchangeable, so determine whether the clause introduced by one of those words is restrictive or non-restrictive, and whether the information in the clause is essential to the meaning of the sentence or provides additional (but non-essential) information.
Beverly Ehrman edits fiction and nonfiction, magazine and newspaper articles, book proposals, educational and promotional materials, business documents, and website copy.