comma with nonrestrictive clauses
The commas that set off a clause adding information about a noun already identified.
Set off a nonrestrictive clause with commas: The brief, which ran to forty pages, was late. When the clause ends the sentence, one comma suffices: The clerk opened the door, which had been locked all night. Do not set off a restrictive clause: The brief that ran to forty pages was late. When a nonrestrictive clause falls in the middle of a sentence, include a comma both before and after the clause, because without the closing comma, the reader cannot tell where the clause ends. See also restrictive / nonrestrictive clauses.
Examples
UseThe night clerk, who had the key, opened the door. (the night clerk already identifies him, so the clause adds nonessential information and takes commas)
UseThe clerk who had the key opened the door. (the clerk alone does not say which clerk, so the clause identifies him and takes no commas)
AvoidThe clerk who had the key, opened the door. (a lone comma separates the subject from its verb)
AvoidThe brief, which ran to forty pages was late. (the closing comma after pages is missing, so the clause has no clear end)