Commas with Additional Information
Use a comma if the additional information is not part of the main statement.
Example: Thank you, ladies and gentlemen, for giving me the opportunity to speak to you today.
Depending on the importance attached to it, additional information can be enclosed in brackets, commas or dashes.
Brackets – not important
Connor (Amy's boyfriend) bought the tickets.
Commas – neutral
Connor, Amy's boyfriend, bought the tickets.
Dashes – emphasised
Connor–Amy's boyfriend–bought the tickets.
Use a comma in relative clauses before who and which if the information is not essential for the understanding of the sentence.
Example: Her brother, who lives in Chicago, came to see her.She has only one brother. He lives in Chicago and came to see her.
Don’t use a comma in relative clauses if the information is essential for the understanding of the sentence.
Example: Her brother who lives in Chicago came to see her. She has more than one brother. But she was visited by only one of them–the brother who lives in Chicago.
Don’t use a comma if the relative clause starts with that.
Example: The book that I’m reading now is interesting.
see also: → Relative Clauses