An ordered pair is two quantities—usually written as (a, b)—that have a significant order; thus, (a, b) does not equal (b, a). Ordered pairs are used in set theory to define members in a function.

Ordered pairs are also valuable in linear equations and graphing, in which the x coordinate is the first number and the y coordinate is the second number, or (x, y). They are used on a grid to locate a point. (For more information about ordered pairs and graphs, see “Geometry and Trigonometry.”)