<P> The coordinates are usually written as three numbers (or algebraic formulas) surrounded by parentheses and separated by commas, as in (3, - 2.5, 1) or (t, u+v, π / 2). Thus, the origin has coordinates (0, 0, 0), and the unit points on the three axes are (1, 0, 0), (0, 1, 0), and (0, 0, 1). </P> <P> There are no standard names for the coordinates in the three axes . The coordinates are often denoted by the letters X, Y, and Z (or x, y, and z), in which case the lines are called the X -, Y -, and Z - axis, respectively . Then the coordinate planes can be referred to as the XY -, YZ -, and XZ - planes . </P> <P> In mathematics, physics, and engineering contexts, the first two axes are often defined or depicted as horizontal, with the third axis pointing up . In that case the third coordinate may be called height or altitude . The orientations are usually chosen so that the 90 degree angle from the first axis to the second axis looks counter-clockwise when seen from the point (0, 0, 1); a convention that is commonly called the right hand rule . </P> <P> A Euclidean plane with a chosen Cartesian system is called a Cartesian plane . Since Cartesian coordinates are unique and non-ambiguous, the points of a Cartesian plane can be identified with pairs of real numbers; that is with the Cartesian product R 2 = R × R (\ displaystyle \ mathbb (R) ^ (2) = \ mathbb (R) \ times \ mathbb (R)), where R (\ displaystyle \ mathbb (R)) is the set of all reals . In the same way, the points in any Euclidean space of dimension n be identified with the tuples (lists) of n real numbers, that is, with the Cartesian product R n (\ displaystyle \ mathbb (R) ^ (n)). </P>

What is the x and y axis called