From Grammar Girl:
A complement—that’s m-e-n-t—is a noun that completes meaning (1). Take the sentence “She is my sister.†The words “my sister†are the complement. There’s no agreement problem in that sentence, it’s all singular, but what about a sentence like this: “The two girls ate their sandwichâ€? Does this mean the girls shared one sandwich or did they each have a sandwich?
