Final answer:
The horizontal number line on the coordinate grid is called the x-axis.
Step-by-step explanation:
In a coordinate grid, the horizontal number line is called the x-axis.
It is represented by a straight line with negative slope on the graph. When two physical quantities are plotted against each other, the horizontal axis is usually considered the independent variable.
For example, on the graph y = mx + b, the x-axis is the horizontal line.