# Ternary relation

In mathematics, a ternary relation or triadic relation is a finitary relation in which the number of places in the relation is three. Ternary relations may also be referred to as 3-adic, 3-ary, 3-dimensional, or 3-place.

Just as a binary relation is formally defined as a set of pairs, i.e. a subset of the Cartesian product A × B of some sets A and B, so a ternary relation is a set of triples, forming a subset of the Cartesian product A × B × C of three sets A, B and C.

An example of a ternary relation in elementary geometry is the collinearity of points.

## Examples

### Binary functions

Further information: Graph of a function and binary function

A function ƒ: A × BC in two variables, taking values in two sets A and B, respectively, is formally a function that associates to every pair (a,b) in A × B an element ƒ(ab) in C. Therefore its graph consists of pairs of the form ((a, b), ƒ(a, b)). Such pairs in which the first element is itself a pair are often identified with triples. This makes the graph of ƒ a ternary relation between A, B and C, consisting of all triples (a, b, ƒ(a, b)), for all a in A and b in B.

### Cyclic orders

Main article: Cyclic order

Given any set A whose elements are arranged on a circle, one can define a ternary relation R on A, i.e. a subset of A3 = A × A × A, by stipulating that R(a, b, c) holds if and only if the elements a, b and c are pairwise different and when going from a to c in a clockwise direction one passes through b. For example if A = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 } represents the hours on a clock face, then R(8, 12, 4) holds and R(12, 8, 4) does not hold.

### Betweenness relations

Main article: Betweenness relation

### Congruence relation

Main article: Congruence modulo m

The ordinary congruence of arithmetics

$a \equiv b \pmod{m}$

which holds for three integers a, b, and m if and only if m divides a - b, formally may be considered as a ternary relation. However, usually, this instead is considered as a family of binary relations between the a and the b, indexed by the modulus m. For each fixed m, indeed this binary relation has some natural properties, like being an equivalence relation; while the combined ternary relation in general is not studied as one relation.

### Typing relation

A typing relation $\Gamma\vdash e\!:\!\sigma$ indicates that $e$ is a term of type $\sigma$ in context $\Gamma$, and is thus a ternary relation between contexts, terms and types.