什么是 Coordinate system？

A coordinate system of the plane consists of an origin $O$ and two graduated axes through $O$: the horizontal $x$-axis and the vertical $y$-axis. Every point $M$ then has a unique pair of coordinates $(x, y)$: its abscissa $x$ and its ordinate $y$. The system is orthonormal when the axes are perpendicular and carry the same unit of length. All systems in this chapter are orthonormal.