A horizontal direction is one that runs parallel to the horizon or the ground. It is a line that runs from left to right across the page. In general, something that is horizontal can be drawn from left to right (or right to left), such as the x-axis in the Cartesian coordinate system. In geometry, a horizontal line is one which runs from left to right across the page. When we graph lines, a horizontal line is a line extending from left to right. Therefore, a horizontal direction is perpendicular to the vertical direction, which runs up and down.