Use a comma (or a pair of commas) to set off elements that are nonessential (also called nonrestrictive). A phrase or clause is nonessential if it can be removed without changing the main idea of the sentence; a nonessential element just adds a relatively unimportant detail. Essential (or restrictive) elements, on the other hand, are not set off by commas because they are too important to be removed from a sentence.
Nonessential participial phrases describe nouns, but the information they provide about those nouns is not very important. Use a comma (or a pair of commas) to separate a nonessential participial phrase from the rest of the sentence.
Walking slowly, the tourists followed the guide through the museum.
The tourists, walking slowly, followed the guide through the museum.
If you remove the participial phrase walking slowly from either of these sentences, the main idea of the sentence (the tourists followed the guide through the museum) doesn't change.
- A participial phrase at the beginning of a sentence is always nonessential.
Essential (or restrictive) participial phrases also describe nouns, but they tell us vital information about those nouns. Do not use commas around essential participial phrases.
The tourists eating ice cream cones were not allowed into the museum until they finished eating.
Only the tourists eating ice cream cones were not allowed in. If you removed the phrase eating ice cream cones, the meaning of the sentence would change—it would mean that none of the tourists was let in, which is very different from the original sentence.
My cell phone, in its blue case, seems to vanish into my backpack whenever it rings. (prepositional phrase)
We don't need to know that the cell phone is in a blue case. This is a nonessential detail.
My cell phone, ringing loudly, has vanished into my backpack. (participial phrase, which is a type of verbal.)
The cell phone would have vanished into your backpack regardless of whether it had been ringing or not, so ringing loudly is a nonessential detail.
Do not use commas to separate essential adjective phrases from the rest of the sentence.
The television with the broken screen needs to be recycled.
The phrase with the broken screen specifies which television. It's not the new television that needs to be recycled; it's the one with the broken screen.
The cell phone ringing loudly needs to be turned off.
Not just any phone needs to be turned off; it's the one that's ringing loudly.
Use commas to separate nonessential appositives from the rest of the sentence.
My Latin teacher, Mr. Virga, gives us homework every night.
We don't need to know your Latin teacher's name to understand that he gives you homework, so his name is a nonessential appositive.
Do not use commas to separate essential appositives from the rest of the sentence.
My friend Elizabeth is one of the nicest people I know.
If you didn't tell us your friend's name, we wouldn't know which friend you were talking about, so her name is an essential appositive.
Use commas to set off nonessential adjective clauses. In American English many authors use which to start nonessential clauses and that to start essential clauses.
Joey ordered frozen yogurt, which he thinks is healthier, instead of ice cream.
The main idea of this sentence is that Joey ordered frozen yogurt instead of ice cream, so it's possible to remove the adjective clause without changing the meaning of the sentence.
It's not necessary to set off an essential adjective clause with commas.
The painting that my mom bought in France is still waiting to be framed.
Without the adjective clause, we are left wondering exactly which painting is waiting to be framed.