When to use which versus that in a sentence is a grammar rule that seems to puzzle even the best of science writers. It seems to me that more often than not in papers, writers pick one at random for each sentence.
There IS actually a rule for when to use which versus that, though…
But don’t worry, we have you covered!
Here’s is everything you need to know for when when to use which versus that in a sentence, complete with a short grammar video!
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Which versus that: the importance test
The use of which versus that comes down to one basic question: how important is this clause to the meaning of your sentence?
To determine the importance:
- Remove the which/that clause from your sentence
- Ask if the meaning of the sentence has changed
Does the meaning STAY THE SAME?
Then this clause is SECONDARY. It provides additional, helpful information, but is not required for the overall meaning.
Does the meaning CHANGE?
Then this clause is CRUCIAL for the meaning of the sentence.
When to use THAT
Any clause that is CRUCIAL to the meaning of the sentence or that would change the meaning of the sentence if removed needs to start with “that”.
To help with commas, clauses starting with “that” are NOT set off from the sentence, so they are not set off using commas.
When to use WHICH
Any clause that is SECONDARY and can be removed without changing the meaning of the sentence needs to start with “which”.
For commas here, since this clause is an aside, it is set off from the rest of the sentence using commas. You will need to put a comma before the word “which” and at the end of the clause if it does not end the sentence.
Examples of when to use WHICH versus THAT
Here is an example sentence:
“The cells [which/that] were treated fluoresced.”
So, let’s start with the importance test and remove this clause to see if the meaning changes:
“The cells fluoresced.”
Here, removing this clause absolutely changed the meaning of the sentence, because not all of the cells here fluoresced – it is important that the reader know it was treated cells that fluoresced. This clause is CRUCIAL.
Therefore, this sentence would use THAT and WOULD NOT be set off by commas:
“The cells that were treated fluoresced.”
Let’s expand this example:
“The cells that were treated fluoresced [which/that] indicated the dye is permeable.”
Now, using the importance test to remove this clause would give us our original example, and the meaning here would not be changed. That means this clause is SECONDARY.
Therefore, this clause would use WHICH and WOULD be set off by commas.
“The cells that were treated fluoresced, which indicated the dye is permeable.”
For one final example to show you what to do when the WHICH clause is in the middle of the sentence:
Let’s take the original sentence:
“The cells that were treated fluoresced and survived longer than untreated cells.”
Now, we want to insert the clause:
“, which indicated the dye is permeable”
To do this, we need to remember to also add a comma at the end of the clause to completely set it off from the rest of the sentence.
“The cells that were treated fluoresced, which indicated the dye is permeable, and survived longer than untreated cells.”