Obligatory and Optional Phrases
A clause, phrase, or word is called restrictive when it’s necessary in order for the sentence to work. Take it out, and the sentence doesn’t convey the same thing.
- Animals such as bears should be treated with caution.
- People who jump queues should be strung up by their toes.
- The dragons fighting above my bed are keeping me awake.
Notice the lack of commas in these examples. Restrictive phrases are never set off with commas.
A clause, phrase, or word is called nonrestrictive when it’s not necessary for the sentence. You could take it out without losing any essential information.
- Hibernating animals, such as bears, should be treated with caution.
- Villanelle, who jumps queues, should be strung up by her toes.
- The castle’s pair of dragons, fighting above my bed, are keeping me awake.
Nonrestrictive phrases, unlike restrictive phrases, are set off with commas.
- The Earl, however, refused to be seen in a Volkswagen.
- The double-action shotgun, not diplomacy, was Anika’s forte.
- The giant Madagascar hissing cockroach, I understand, makes an affectionate pet.
Because of this, adding or deleting commas can change the information in a sentence.
- Her romance novel, Scottish Lords with Bulging Calf Muscles, is a bodice-ripper.
Commas tell us the title is nonrestrictive: you could take it out and the sentence would say the same thing. Which means this is her only romance novel.
- Her romance novel Scottish Lords with Bulging Calf Muscles is a bodice-ripper.
The absence of commas tells us the title is restrictive: without it we wouldn’t know which romance novel was meant. Which suggests she has written other romance novels, some of which may feature other kinds of lords with different muscles.
That said, the current preference among writers and editors seems to be for fewer commas, and as long as leaving out the commas isn’t likely to confuse the reader, then it’s acceptable to do so even with a nonrestrictive phrase. For example, if you leave out the commas around Myrna in my wife Myrna loves taxidermy, your readers probably won’t assume you have more than one wife.
On the other hand, if you’re writing about, say, siblings or offspring, you might want to use commas to show you only have one: our son, Xiaoping; my sister, Susan. (For more on this, see Commas, Names, and Chopsticks.)
That vs. Which
Understanding the difference between restrictive and nonrestrictive clauses is essential for choosing correctly between that and which. According to the that/which rule, that belongs in restrictive clauses and which belongs in nonrestrictive clauses.
- The UFO that arrived last week is still parked on our lawn. (Not to be confused with the UFO that arrived the week before.)
- The UFO, which arrived last week, is still parked on our lawn. (Only one alien craft has ever landed here.)
In UK-style English, which is as likely to be used in a restrictive clause as that.
- UK: The dark cloud which hung over the mansion put a pall on their croquet game.
- US: The dark cloud that hung over the mansion put a pall on their croquet game.
But in any country, nonrestrictive clauses always use which.
- UK & US: The corpse, which was oddly familiar, gave Teuta pause.
Do You Need That That?
The that in a restrictive clause can sometimes be left out if the meaning of the sentence is clear without it.
- Desta told him that it was time.
- Desta told him it was time.
- If I had known that the plant was carnivorous, I wouldn’t have bought it.
- If I had known the plant was carnivorous, I wouldn’t have bought it.
- Hari realized that that was the witch’s plan all along.
- Hari realized that was the witch’s plan all along.
Eliminating superfluous thats can make your writing cleaner and easier to read, which is why some editors insist on deleting the word wherever it turns up; however, there are places where a that is necessary for clarity.
- I learned when she took her camera I was not to follow.
- I learned that when she took her camera I was not to follow.
- I learned when she took her camera that I was not to follow.
It’s good to question your thats, because we often overuse them without realizing it, but don’t assume any that is a bad that.
In summary, use that when the clause is necessary to the sentence, and use which when it’s not. Unless you’re following UK style, in which case you can use which for necessary clauses too. But either way, when your clause is unnecessary, use which—and don’t forget the commas.
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