A preorder (also sometimes called a quasi-order, especially if one works with <\lt instead of \leq) is like a partial order, but without the “antisymmetry” requirement that xyx \leq y and yxy \leq x implies x=yx = y.

By interpreting the relation \leq as the existence of a unique morphism, preorders may be regarded as certain categories (namely, thin categories). This category is sometimes called the preorder category associated to a preorder; see below for details.


A preorder on a set SS is a reflexive and transitive relation, generally written \leq. A preordered set, or proset, is a set equipped with a preorder. (This should not be confused with a pro-set, i.e. a pro-object in Set.)

Equivalently, a proset is a (strict) thin category: a strict category such that for any pair of objects x,yx, y, there is at most one morphism from xx to yy. The existence of such a morphism corresponds to the truth of the relation xyx\leq y. In other words, it’s a (strict) category enriched over the cartesian monoidal category of truth values.

In homotopy type theory

A preorder AA in homotopy type theory consists of the following:

  • A homotopy type A 0A_0, whose terms a:A 0a:A_0 we write as a:Aa:A.
  • For every a,b:Aa, b:A, a proposition p:a Abp: a \leq_A b
  • For every a:Aa:A, a proposition refl a:a Aarefl_a: a \leq_A a called reflexivity
  • For every a,b,c:Aa, b, c:A, a function
    (a Ab)(b Ac)(a Ac)(a \leq_A b) \rightarrow (b \leq_A c) \rightarrow (a \leq_A c)

    called transitivity, and denoted infix as an implication by pqpqp \implies q \implies p \wedge q

  • For every a,b:Aa,b:A, and p:abp: a \leq b we have prefl app \iff refl_a \wedge p and pprefl bp \iff p \wedge refl_b
  • For every a,b,c,d:Aa,b,c,d:A and p:abp:a\leq b, q:bcq:b\leq c, r:cdr:c\leq d, we have p(qr)(pq)rp \wedge (q \wedge r) \iff (p \wedge q) \wedge r.

A proposition p:abp:a \leq b is an equivalence relation if there is a proposition q:baq:b \leq a such that pqrefl ap \wedge q \iff refl_a and qprefl bq \wedge p \iff refl_b. We write a~ Aba ~_A b for the type of such equivalence relation, which is also a proposition.

The only relationship between the equivalence relation a~ Aba ~_A b as defined and the identity type Id A(a,b)Id_A(a, b) for a,b:Aa,b:A is that there exists a function

idtoeq a,b:Id A(a,b)a~ Abidtoeq_{a,b}: Id_A(a,b) \rightarrow a ~_A b

A preordered set or proset is a preorder such that the type A 0A_0 is 0-truncated, and a partial order is a preorder such that for every a,b:Aa,b:A, the function idtoeq a,bidtoeq_{a,b} is a homotopy equivalence. A partially ordered set or poset is a preordered set and a partial order. Every set is the core of a proset.


Relation to partial orders

Any preordered set is equivalent to a poset. This is a special case of the theorem that every category has a skeleton, but (if you define ‘equivalence’ weakly enough) this case does not require the axiom of choice.

If the foundations have quotient sets, then every preorder has a quotient set equivalent to a poset. Let (P,)(P, \leq) be a preorder. Define the equivalence relation \sim on PP for all elements a,bPa, b \in P as ab:=(ab)(ba)a \sim b := (a \leq b) \wedge (b \leq a). Then the quotient set P/P / \sim is a poset. This is the 0-truncated version of the fact that because every precategory has a core pregroupoid and every pregroupoid has a Rezk completion into a groupoid, every precategory has a Rezk completion into a category.

Relation to thin categories

In set-theoretic foundations, a preordered set is the same as a thin category (a category in which any two parallel morphisms are equal), and it is partially ordered just when it is skeletal. Thus, asking for a preordered set to be partially ordered may seem to break the principle of equivalence of category theory. However, as remarked above, a thin category always has a skeleton which is a poset; so working with posets up to isomorphism is the same as working with preordered sets up to equivalence. In other words, if xyx \le y and yxy \le x, so that xx and yy are isomorphic, we may as well say that they are equal (since they are isomorphic in only one way).

Another way to say this is that the nerve simplicial set of a preorder, which is necessarily a Segal space or category object in an (infinity,1)-category in SetGrpdSet \hookrightarrow \infty Grpd, is in addition a complete Segal space or genuine category object in SetSet if the preorder is in fact a partial order. For more on this perspective see at Segal space – Examples – In Set.

If we distinguish between isomorphism and equality of elements in a preordered set (hence considering preordered sets up to isomorphism, rather than up to equivalence), then this is equivalent to considering the corresponding thin category to also be a strict category. When treated in this sense, preordered sets are not equivalent to posets.

On the other hand, in non-set-theoretic foundations where not every category need have an underlying set (i.e. need not be a strict category in any canonical way) — such as homotopy type theory or preset theories — a preordered set defined as “a set with a relation \leq …” is automatically a strict category, with a notion of equality of objects coming from the given set. By contrast, in this case a thin category (as opposed to a more general category) does have a canonical structure of strict category in which equality of objects means isomorphism, but not every strict thin category is canonical in this sense. In this case, partially ordered sets correspond to thin categories (with canonical strict-category structures), while preordered sets correspond to thin categories with arbitrary strict-category structures.

Preorder reflection

The 2-category of preorders (more precisely, that of thin categories) is reflective in Cat. The reflector preserves the objects and declares xyx \leq y if there exists an arrow from xx to yy.

Cauchy completion


Internal to any regular category every poset is a Cauchy complete category.

This appears as (Rosolini, prop. 2.1).


Internal to any exact category the Cauchy completion of any preorder exists and is its poset reflection?.

This appears as (Rosolini, corollary. 2.3).


Cauchy completion for preorders is discussed in

  • G. Rosolini, A note on Cauchy completeness for preorders (pdf)

Last revised on September 26, 2021 at 01:42:59. See the history of this page for a list of all contributions to it.