It is the y axis and the horizontal line is the x axis and both axes are perpendicular to each other at the point of the origin
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It is the vertical number line on the y axis in the Cartesian plane
The horizontal line is the x axis and the vertical line is the y axis.
On the Cartesian plane the x line is the horizontal axis and the y line is the vertical axis.
Cartesian plane
The y-axis is the vertical line that is perpendicular to the horizontal line of the x-axis on the Cartesian plane