Which or That?

Explanation: In formal writing, it is more common to use the pronoun which with a comma and that without a comma. That indicates that the information in the adjective clause is essential, while which indicates is just extra information.

Let's look at these two sentences:
Honolulu, which was my hometown, has a vibrant tourist driven economy.
He lives in one of only two states that hold elections from 12 a.m. on election day.

The first sentence has a clause that gives extra information about Honolulu. If you remove it, the sentence still has meaning.
The second sentence has a clause that tells us what states we are talking about. If you remove it, the sentence loses meaning.

We only do this with things.

Remember: Use commas around extra information and no commas when the information is necessary to understand the meaning of the noun and sentence.
Commas/extra: I ran into Sara, who still has not paid me back yet, in that new mall in Norwalk yesterday.
No commas/essential: This morning, I ran into a couple who just moved into that new house two doors down from us.

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