what way is horizontal

A horizontal line is a straight line that runs from left to right or right to left and is parallel to the ground or the horizon. In geometry, a horizontal line is one that runs parallel to the x-axis. The slope of a horizontal line is always 0, which means that it never moves up or down. The word "horizontal" comes from the word "horizon," which is the line where the earth and sky appear to meet far away. In contrast, a vertical line runs up and down and is perpendicular to a horizontal line.