in coordinate geometry i am having problem with the sign convention of directed distances. Let P1 and P2 be arbitrary points on the graph. Then what is the sign convention for P1P2 to be positive or negative. I know that if P1P2 is parallel to x-axis or y-axis then the normal convention for positive and negative direction (right=postive............up=positive). But what happens when P1P2 is not parallel to x axis or y axis. What is the convention to determine whether P1P2 is positive or negative. Please exhaust all the possible cases.