A morphism (or arrow) R : A → B in this category is a relation between the sets A and B, so R ⊆ A × B.
The composition of two relations R: A → B and S: B → C is given by
- (a, c) ∈ S o R ⇔ for some b ∈ B, (a, b) ∈ R and (b, c) ∈ S.
Rel has also been called the "category of correspondences of sets".
A morphism in Rel is a relation, and the corresponding morphism in the opposite category to Rel has arrows reversed, so it is the converse relation. Thus Rel contains its opposite and is self-dual.
The category has two functors into itself given by the hom functor: A binary relation R ⊆ A × B and its transpose RT ⊆ B × A may be composed either as R RT or as RT R. The first composition results in a homogeneous relation on A and the second is on B. Since the images of these hom functors are in Rel itself, in this case hom is an internal hom functor. With its internal hom functor, Rel is a closed category, and furthermore a dagger compact category.
The category Rel was the prototype for the algebraic structure called an allegory by Peter J. Freyd and Andre Scedrov in 1990. Starting with a regular category and a functor F: A → B, they note properties of the induced functor Rel(A,B) → Rel(FA, FB). For instance, it preserves composition, conversion, and intersection. Such properties are then used to provide axioms for an allegory.
Relations as objects
David Rydeheard and Rod Burstall consider Rel to have objects that are homogeneous relations. For example, A is a set and R ⊆ A × A is a binary relation on A. The morphisms of this category are functions between sets that preserve a relation: Say S ⊆ B × B is a second relation and f: A → B is a function such that then f is a morphism.
The same idea is advanced by Adamek, Herrlich and Strecker, where they designate the objects (A, R) and (B, S), set and relation.
- Mac Lane, S. (1988). Categories for the Working Mathematician (1st ed.). New York: Springer-Verlag. p. 26. ISBN 0-387-90035-7.
- Pareigis, Bodo (1970). Categories and Functors. Pure and Applied Mathematics. 39. Academic Press. p. 6. ISBN 978-0-12-545150-5.
- This category is called SetRel by Rydeheard and Burstall.
- George Bergman (1998), An Invitation to General Algebra and Universal Constructions, §7.2 RelSet, Henry Helson Publisher, Berkeley. ISBN 0-9655211-4-1.
- Michael Barr & Charles Wells (1998) Category Theory for Computer Scientists Archived 2016-03-04 at the Wayback Machine, page 83, from McGill University
- Peter J. Freyd & Andre Scedrov (1990) Categories, Allegories, pages 79, 196, North Holland ISBN 0-444-70368-3
- David Rydeheard & Rod Burstall (1988) Computational Category Theory, page 41, Prentice-Hall ISBN 978-0131627369
- Juri Adamek, Horst Herrlich, and George E. Strecker (2004)  Abstract and Concrete Categories, section 3.3, example 2(d) page 22, from Research group KatMAT at University of Bremen
- Francis Borceux (1994). Handbook of Categorical Algebra: Volume 2, Categories and Structures. Cambridge University Press. p. 115. ISBN 978-0-521-44179-7.