With two-dimensional Cartesian coordinates (or rectangular coordinates), an *ordered pair* of two numbers is determined using two axes oriented perpendicular to each other (for more about ordered pairs, see “Foundations of Mathematics”). The ordered pair coordinates are found by specifying points along the two axes: First, along the x-axis, the positive or negative amount on either side of the *y,* or vertical, axis; this is called the *abscissa* and indicated as *x.* The other number is the positive or negative amount on either side of the *x,* or horizontal, axis; this is called the *ordinate* and indicated as *y.* The coordinates are usually written in terms of *x* and *y* at the point as an ordered pair; for example, in the illustration of a two-dimensional coordinate system below, the ordered pair is (7, 8), in which *x* is 7 and *y* is 8.