A coordinate system in which the locations of points in space are expressed by reference to three planes, called coordinate planes, no two of which are parallel. The three planes intersect in three straight lines, called coordinate axes. The coordinate planes and coordinate axes intersect in a common point, called the origin. From any point P in space three straight lines may be drawn, each of which is parallel to one of the three coordinate axes; each of these lines will then intersect one (and only one) of the three coordinate planes. If A, B, C denote these points of intersection, the Cartesian coordinates P are the distances PA, PB, and PC. If the coordinate axes are mutually perpendicular, the coordinate system is rectangular; otherwise, oblique. In meteorology, the most common orientation of the x, y, z rectangular Cartesian coordinates is such that the x axis is directed toward the east, tangent to the earth's surface; the y axis toward the north, tangent to the earth's surface; and the z axis toward the local zenith, perpendicular to the earth's surface. Compare curvilinear coordinates.