# Why are equivalence classes called “classes” and not “sets”?

Why are equivalence classes called so and not equivalence sets? I am kind of not able to find the difference between a class and a set. What properties that a set have that a class cannot have? It looks like classes are introduced to get away from Russel's paradox how does it?

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The use of the word "class" in the phrase "equivalence class" has nothing to do with the use of the word "class" to refer to something that's not allowed to be a set. – Gerry Myerson May 7 '13 at 13:17
Note that class was originally used as a synonym of set before any real attention was paid to the antimonies of naive set theory. I would guess that the term "equivalence class" was already fairly common before a formal distinction between set and class was made, I believe by von Neumann. – arjafi May 7 '13 at 13:20
For the same reason that math class is not called a math set. – vadim123 May 7 '13 at 13:25
Actually, we can define equivalence relations on classes. For example $A\sim B:\Leftrightarrow$ there exists a bijection $A\to B$. The equivalence classes of such a relation are proper classes (with one exception). – Hagen von Eitzen May 7 '13 at 13:25

The Oxford English Dictionary s.v. equivalence quotes Kleene, Introduction to Metamathematics ($1952$), as follows:

Von Neumann $1928$ chooses from each of these sets of sets (‘equivalence classes’ [Ger. Äquivalenzklassen]) a particular set to serve as the cardinal of any set in the class.

This is the earliest citation that clearly refers to the usual modern sense of the term. It’s possible, then, that the English term is simply a calque of the German, and that the question should be why the German term uses Klasse rather than Menge.

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In this specific application, the Klasse is a proper Klasse and not a Menge as these "sets of sets" aren't really sets, are they? – Hagen von Eitzen May 7 '13 at 13:31
@Hagen: Yes, but it’s not clear to me that this is relevant: I’m not at all certain that that distinction was already firmly in place at the time. – Brian M. Scott May 7 '13 at 13:32
+1 for "calque" :) [New to me!] – Peter Smith May 7 '13 at 13:54

The term "equivalence class" is a single term in the mathematical language. It does, however, relate to the term "class" from set theory.

Classes, in modern set theory, are collections which are defined by a formula (perhaps with parameters). An equivalence class is also defined by a formula with parameters. We have two parameters, the equivalence relation and the representative of the equivalence class.

In theories like $\sf ZFC$ every set is a class. Because every set is defined by a formula using itself as a parameter. It's a bit of cheating, but mathematically it is correct. In our case parameters are things we already know are sets, and if $A$ is a set then we can use it as a parameter for the formula $x\in A$.

The difference between proper classes, i.e. classes which are not sets, and sets has been discussed greatly on this site. In a nutshell proper classes are collection we can define (in the language of set theory) but we can prove that they do not form a set. One difference is that while we have a coherent way of assign "size" to a set, we don't have the ability to assign "size" to a proper class.