A *pair* is usually an n-tuple for $n = 2$, that is an *ordered pair*. Such a pair is a term of product type, an element of a Cartesian product, a generalized element of a product object, etc.

But a *pair* may also be an *unordered pair*. The existence of these is a basic axiom of ZFC and its variations, the axiom of pairing.

Last revised on April 6, 2017 at 04:22:09. See the history of this page for a list of all contributions to it.