To take the preceding discussion a step further, consider once again the distance between two points on a line. In Figure 7.1, you establish a relationship between two ordered pairs, (4, 9) and (1, 5). As the figure reveals, the distance between the two points can be viewed in terms of the differences between the corresponding elements of the two ordered pairs. Accordingly, the difference between 9 and 5 is 4 and the difference between 4 and 1 is 3. The figure uses the delta character to show this difference (or change).