Supply and Demand
Mathematical Convention
The mathematical convention is that the independent variable is on the horizontal axis, and the dependent variable is on the vertical axis. In the short run, in a supply-and-demand diagram, it's the other way around -- price, the independent variable, is on the vertical axis. But in the long run, price is the dependent variable. We can't have it both ways, so we just draw the diagrams for the long run and violate the mathematical convention when we discuss the short run.