Usually we encounter plane rectangular coordinates, which are composed of abscissa and ordinate, representing the horizontal distance and vertical distance from the target point to the origin respectively, and are expressed by (x, y).

Polar coordinates are also expressed by two numerical values, one is the degree of the included angle between the connecting line between the target point and the origin and the horizontal line, and the other is the linear distance between the target point and the origin, which is expressed by (θ, a).