Commas can be used to set off
nonessential words,
phrases, and
clauses that interrupt a sentence. A nonessential element provides extra information, but is not necessary to understanding the meaning of a sentence.
Conjunctive adverbs can sometimes be used in the middle or at the end of clauses; when this happens, they should be set off by commas
- The student council will, however, continue investigating the issue of littering on campus.
-
Despite numerous complaints, students are making efforts to meet the new backpack rules enforced by the school, nonetheless.
When people or groups are addressed in the middle or at the end of a sentence, the name used for them should be set off by commas.
- I wonder, class, how everyone will fare on this pop quiz on yesterday's lesson.
- I'm not really sure if I'll have time to go to the movies with you, Jessica.
Sometimes, a name is essential and should not be set off by commas. If the person being referred to is unclear without the name, then the name is essential.
- The American poet Robert Frost won the Pulitzer Prize for poetry four times.
- The American poet (which poet?) won the Pulitzer Prize for poetry four times.
If a noun clearly refers to one person, then a name is extra information and is nonessential.
- My only sister, Claire, and I share a birthday, but she is older than me by five years.
Descriptive words, such as
participles, can sometimes follow after the nouns they describe in a sentence. They are usually considered
nonessential and should be set off by
commas.
- The queen, exasperated, ordered the council meeting done for the day and declared that they would meet again in the morning.
- Tammy, limping, deeply regretted buying her new shoes online instead of in person at a store.
An appositive is a noun or noun phrase that renames or further describes a noun next to it. If the noun is already named specifically or clearly refers to a single person or thing, the appositive is considered nonessential and should be set off by commas.
- Venice, the floating city, was built on 118 small islands in the middle of a lagoon.
- Your notebook, the dark blue one with stars on the cover, is on the table in the back of the library.
A participial phrase contains a participle + an object/modifiers. When a participial phrase is
nonessential, it should be set off by commas.
- The young girls, frightened by the howling wind, ran to their parents' bedroom.
- The professor's hamster, happily rolling around its cage, loved to put on a show for whoever was in the room.
Clauses that begin with "that" are always essential. Commas should not be used.
- The house that had gone unsold for nearly a year finally has new owners.
Clauses that begin with "which" are always nonessential. Commas should be used to set them off from the rest of a sentence.
- Alison only wears that necklace, which has been in her family for generations, on special occasions.
Clauses that provide extra information, but are not necessary to understanding the sentence, are considered to be nonessential and should be set off by commas.
- My best friend's aunt, who catered the Christmas party, was recently on the cover of a cooking magazine.