### Theory:

Number line:

In the above number line, the position of point $$P$$ can be represented as $$6$$ units with the reference of origin to one line (horizontal line).

A similar rule applies to the negative side of the number line as well. It is possible to represent the position of a point with reference to more than one point.

Descartes invented the idea of placing two such number lines perpendicular to each other on a plane and locating points on the plane by referring them to these lines.

The perpendicular lines may be in any direction. But, when we choose these two lines to locate a point in a plane in this chapter, one line will be horizontal, and the other will be vertical, as below.

Cartesian system:
• It is a co-ordinate number system which is used to describe the position of a point in two dimensions($$x$$-axis and $$y$$-axis).
• The representation of the position of a point in a number line will be the distance of the point from the origin in the $$x$$-axis, the distance of the point from the origin in the $$y$$-axis.
Co-ordinate axes:
The plural form of axis is called the axes. A number line represented horizontally is the $$x$$-axis and a number line represented vertically is the $$y$$-axis. Joining both the lines together at origin is called the $$xy$$ plane or cartesian plane or co-ordinate axes.