When we’re talking about the relative pronouns that and which, we need to be careful which ones we use. This mistake again falls in with those that I find are very common in manuscripts of all types.
That clauses are called restrictive, meaning they are essential to the meaning of the sentence. For example, “The objects that were valuable were kept and stored.” That were valuable is essential to the meaning of the sentence.
Which clauses, however, can simply add information and are then considered nonrestrictive, or they can be essential to the meaning of the sentence and are then considered restrictive. For a nonrestrictive example, “The objects, which were considered valuable at one time, were no longer thought to be valuable and were discarded.” You can take out the clause “which were considered valuable at one time,” read the sentence without it, and the sentence still makes sense. It’s merely adding additional information.
In the restrictive sense, the writer removes the commas that set off the nonrestrictive clause and the meaning changes a bit, “The objects which were considered valuable at one time were no longer thought to be valuable and were discarded.” APA style prefers the use of that when the meaning is meant to be restrictive.
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