The plane rectangular coordinate system has two coordinate axes, in which the horizontal axis is the X axis and the right direction is the positive direction; The vertical axis is the y axis and the direction is the positive direction.

Two number axes perpendicular to each other on the same plane and having a common origin form a plane rectangular coordinate system, which is called rectangular coordinate system for short. Usually, the two number axes are placed in horizontal and vertical positions respectively, and the right and upward directions are the positive directions of the two number axes respectively. The horizontal axis is called X axis or horizontal axis, the vertical axis is called Y axis or vertical axis, and the X axis and Y axis are collectively called coordinate axes. Their common origin O is called the origin of rectangular coordinate system, and the plane rectangular coordinate system with point O as the origin is recorded as the plane rectangular coordinate system xOy.