 In Quine's set-theoretical writing, the phrase "ultimate class" is often used instead of the phrase "proper class" emphasising that in the systems he considers, certain classes cannot be members, and are thus the final term in any membership chain to which they belong.Outside set theory, the word "class" is sometimes used synonymously with "set".This usage dates from a historical period where classes and sets were not distinguished as they are in modern set-theoretic terminology.