**Angle of Inclination of
a Line**

The angle between
a line and the *x*-axis.
This angle is always between 0° and 180°, and is
measured counterclockwise from
the part of the *x*-axis
to the right of the line.

Note: All horizontal lines
have angle of inclination 0°. All vertical lines
have angle of inclination 90°. Also, the slope
of a line is given by the tangent of
the angle of inclination.