A horizontal line is a line that is parallel to the x-axis of the coordinate plane.
The general equation of a horizontal line is y=c where c is a constant. In y=c, it implies that the y-value of the horizontal line is the same or never changes. However, the x-value of the horizontal line can take any values.
If you take any two points of a horizontal line and find the slope, it will always equal to zero. That is why all horizontal lines have a slope of zero, m=0.
Example: Graph y=3.
Since the y-value doesn’t change, we can describe at least five points or ordered pairs that lie on the horizontal line, y=3 which can be (-2,3), (-1,3), (0,3), (1,3), and (2,3). Plotting these points on the coordinate plane reveals a horizontal line.