{ } In order to discuss all the subsets of a given set, let us introduce the following terminology. We shall call the original set the universal set, one-element subsets will be called unit sets, and the set which contains no members the empty set. We do not introduce special names for other kinds of subsets of the universal set. As an example, let the universal set U consist of the three elements {a, b, c}. The proper subsets of U are those sets containing some but not all of the elements of U. The proper subsets consist of three two-element sets namely, {a, b}, {a, c}, and {b,.