Anyone who has read my writing tips has noticed I have a strong aversion to the word "that" in writing. I have found it to be a superfluous word and unworthy of inclusion in most sentences.
BUT, I am willing to concede to its usage, or non-usage, when in the proper context.
People are constantly confusing "that" and "which" within a sentence, thinking they are interchangeable. The words are NOT interchangeable and do not mean the same thing.
Jewelry that glitters often catches a person's eye.
First, we can't remove "that" from the sentence. It obviously is not superfluous.
Second, the sentence is restrictive because you can't remove "that glitters" and have the same meaning. What does this mean? If the sentence was "Jewelry often catches a person's eye" — it isn't the same since it means any and all jewelry does this. The restrictive aspect — that glitters — selects only those pieces of jewelry which glitter to be noticed.
Therefore, "that" is used in restrictive clauses to validate and clarify a sentence's meaning.
Diamond jewelry, which is expensive, often catches a person's eye.
"Which" is used to introduce a "nonrestrictive clause" that can be left out and not change the sentence's meaning. By removing "which is expensive" the sentence "Diamond jewelry often catches a person's eye" has not changed it meaning. We no longer know diamonds are expensive, but again, it is not important to the sentence.
Of course, one could say "which" clauses are always set off by commas, but, then again, this would not be true. Consider the following sentence:
By removing the last three words (which is bad) the sentence has not lost its meaning nor impact.
A simple rule of thumb: