supplementary relative clause

— Supplementary relative clauses (which other books call “appositive relatives” or “non-restrictive relatives”) are optional parenthetical interruptions of sentences. They can always be left out without changing the main assertion (though the ancillary information they contribute will be gone). The most important thing to remember about them in writing is that they must be separated off by punctuation, typically commas: The duke, [who the organizers had hoped __ would present the award], was unable to attend owing to illness.