A derived binary relation between two sets is the subset relation, also called "set inclusion". If all the members of set are also members of set , then is a "subset" of , denoted . For example, is a subset of , and so is but is not. As implied by this definition, a set is a subset of itself. For cases where this possibility is unsuitable or would make sense to be rejected, the term "proper subset" is defined. is called a "proper subset" of if and only if is a subset of , but is not equal to . Also, 1, 2, and 3 are members (elements) of the set , but are not subsets of it; and in turn, the subsets, such as {1}, are not members of the set {1, 2, 3}.