Definition:Ordered Pair/Informal Definition

Definition
The definition of a set does not take any account of the order in which the elements are listed.

That is, $\set {a, b} = \set {b, a}$, and the elements $a$ and $b$ have the same status -- neither is distinguished above the other as being more "important".

An ordered pair is a two-element set together with an ordering.

In other words, one of the elements is distinguished above the other - it comes first.

Such a structure is written:
 * $\tuple {a, b}$

and it means:
 * first $a$, then $b$.

Also see

 * Equivalence of Definitions of Ordered Pair