Definition:Doubleton

Definition
A doubleton is a set that contains exactly two elements.

The doubleton containing the distinct elements $a$ and $b$ can be written $\set {a, b}$.

The set $\set {a, b}$ is known as the doubleton of $a$ and $b$.

Also known as
Doubletons are also frequently referred to as unordered pairs, or even simply pairs if no ambiguity results.

Also see

 * Axiom:Axiom of Pairing


 * Union of Disjoint Singletons is Doubleton for a proof from the ZF axioms that $\set a \cup \set b = \set {a, b}$ when $a \ne b$.