A horizontal line is parallel to the x-axis and is perpendicular, as well as intersects, the y-axis. The slope of a horizontal line is always zero, as its change in y is always zero. A vertical line, on the other hand, is parallel to the y-axis and is perpendicular, as well as intersects, the x-axis. The slope of a vertical line is undefined, as its change in x is always zero, thus giving a slope with zero in the denominator. See how Stephanie applies these properties to plot movements for her game character 8-BitBot in navigating vertical and horizontal direction in her computer gaming program. Common Core Reference: CCSS.MATH.CONTENT.8.FA.3