[Aside: When I was studying Russian in college, for some reason, I really struggled with the partitive genitive–not in English, but in Russian. If you have children or students studying inflected languages, this squib might be of some assistance to them.]
Partitive: Indicating restricting, setting off, or only a part of, e.g. ‘a scrap of food,’ ‘one of your friends.’”
Excerpted from: Grambs, David. The Random House Dictionary for Writers and Readers. New York: Random House, 1990.