          # Ordered pair

• in mathematics, an ordered pair (a, b) is a pair of objects. the order in which the objects appear in the pair is significant: the ordered pair (a, b) is different from the ordered pair (b, a) unless a = b. (in contrast, the unordered pair {a, b} equals the unordered pair {b, a}.)

ordered pairs are also called 2-tuples, or sequences (sometimes, lists in a computer science context) of length 2; ordered pairs of scalars are also called 2-dimensional vectors. the entries of an ordered pair can be other ordered pairs, enabling the recursive definition of ordered n-tuples (ordered lists of n objects). for example, the ordered triple (a,b,c) can be defined as (a, (b,c)), i.e., as one pair nested in another.

in the ordered pair (a, b), the object a is called the first entry, and the object b the second entry of the pair. alternatively, the objects are called the first and second components, the first and second coordinates, or the left and right projections of the ordered pair.

cartesian products and binary relations (and hence functions) are defined in terms of ordered pairs.

• generalities
• informal and formal definitions
• defining the ordered pair using set theory
• category theory

## In mathematics, an ordered pair (a, b) is a pair of objects. The order in which the objects appear in the pair is significant: the ordered pair (a, b) is different from the ordered pair (b, a) unless a = b. (In contrast, the unordered pair {a, b} equals the unordered pair {b, a}.) Ordered pairs are also called 2-tuples, or sequences (sometimes, lists in a computer science context) of length 2; ordered pairs of scalars are also called 2-dimensional vectors. The entries of an ordered pair can be other ordered pairs, enabling the recursive definition of ordered n-tuples (ordered lists of n objects). For example, the ordered triple (a,b,c) can be defined as (a, (b,c)), i.e., as one pair nested in another. In the ordered pair (a, b), the object a is called the first entry, and the object b the second entry of the pair. Alternatively, the objects are called the first and second components, the first and second coordinates, or the left and right projections of the ordered pair. Cartesian products and binary relations (and hence functions) are defined in terms of ordered pairs. Contents 1 Generalities 2 Informal and formal definitions 3 Defining the ordered pair using set theory 3.1 Wiener's definition 3.2 Hausdorff's definition 3.3 Kuratowski's definition 3.3.1 Variants 3.3.2 Proving that definitions satisfy the characteristic property 3.4 Quine–Rosser definition 3.5 Cantor–Frege definition 3.6 Morse definition 4 Category theory 5 References  