That vs Which

While both “that” and “which” are used without problem in other constructions, using them as relative pronouns seems to cause some confusion.

The rule of thumb* is to use “that” before essential clauses and “which” before nonessential clauses. In both cases, use “who” when the subject is a person.

An essential clause must be included in the sentence to specify the subject: The trees that were pruned have lots of blossoms.  Using “that” specifies that only some of the trees were pruned and only those have lots of blossoms.

A nonessential clause can be omitted without changing the meaning of the sentence. Commas set it off from the rest of the sentence: The trees, which were pruned, have lots of blossoms. Using “which” tells us that all the trees were pruned and all have lots of blossoms. Omitting the clause results in “The trees have lots of blossoms,” which retains the meaning.

*Note that British usage is different and that some insist the rules need to be more flexible—and complex.

Joy Eckel, Freelance Copy Editor
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