Speaking of which, I’d like to present a grammar lesson: That or Which?
These pronouns both introduce clauses, but they shouldn’t be used interchangeably. Knowing which is which is not that difficult.
That is used to introduce essential clauses about an inanimate object. Which is primarily used to introduce nonessential clauses about an inanimate object. Let’s look at a couple of examples:
Horror movies that were directed by Wes Craven are playing at the downtown theater.
Horror movies, which can be traumatic for young viewers, are playing at the downtown theater.
In the first sentence, the underlined clause tells us which movies are playing. Not all horror movies are playing downtown — only specific ones. The clause is essential to complete the sentence’s meaning, so no commas are needed.
In sentence two, the clause about trauma adds incidental information about the movies that are playing. It is not essential and must be set off by commas.
Does that make sense? Let’s try two more examples, sticking with our cinematic theme:
Tickets that are purchased online are generally less expensive.
Tickets, which can be purchased online, are available.
The underlined clause in the first example restricts the meaning — it tells us the specific tickets that are less expensive. We need this information. If we eliminate those four words, we change the meaning of the sentence. So, no commas.
Those five words that make up the clause in the second example, however, can be eliminated without changing the basic meaning of the sentence. They merely provide additional detail, and their absence would not lead to misinterpretation. So, we need commas.
That concludes today’s lesson. That’s that. (Or is that which?)