Two sets can also be "subtracted". The relative complement of B in A (also called the set-theoretic difference of A and B), denoted by A B (or A − B), is the set of all elements that are members of A but not members of B. Note that it is valid to "subtract" members of a set that are not in the set, such as removing the element green from the set {1, 2, 3}; doing so has no effect.