The rise of a road between two points is the difference between the altitude of the road at those two points, say "y"1 and "y"2, or in other words, the rise is ("y"2 − "y"1) = Δ"y". For relatively short distances, where the earth's curvature may be neglected, the run is the difference in distance from a fixed point measured along a level, horizontal line, or in other words, the run is ("x"2 − "x"1) = Δ"x". Here the slope of the road between the two points is simply described as the ratio of the altitude change to the horizontal distance between any two points on the line.