The ___ is the vertical number line on a coordinate plane.

The vertical number line on a coordinate plane is called the "y-axis." It runs up and down and is used to measure the vertical position of points in a two-dimensional space. When graphing, the y-axis is essential for identifying the second coordinate in an ordered pair (x, y), helping us locate points in relation to both the x-axis and one another. While using the y-axis, common mistakes include confusing it with the x-axis, which runs horizontally. Additionally, it's important to note that the values on the y-axis increase as you move upwards and decrease downwards. Always double-check that your points are placed accurately according to these rules to prevent misinterpretation of your graph!