Raise the price and sellers make more but buyers want less; lower it and the reverse. The market’s one trick is to find the single price where those two wants exactly meet — the equilibrium. Push the price away from it and a gap opens: a glut, or a shortage. Slide the price and watch the gap.
Linear curves: supply S(p)=a+b·p (rises with price), demand D(p)=c−d·p (falls). They cross at the equilibrium price p* = (c−a)/(b+d), where the quantity supplied exactly equals the quantity demanded and the market clears. Set a price below p* and demand outruns supply — a SHORTAGE (D−S>0); above p*, a glut. A fail-loud self-check throws unless S(p*)=D(p*) exactly and a below-equilibrium price opens a positive shortage — the mechanism behind every cleared market and every price-control queue.
Straight-line curves and a single market are the textbook idealisation; real curves bend, shift, and interact (elasticity, shocks, expectations). The equilibrium arithmetic and the shortage a ceiling creates are exact.