Supply and demand begins with a benchmark

Mark the original equilibrium before changing a curve.

Place price vertically and market quantity horizontally. Demand slopes down. Supply slopes up. Mark their intersection and project dashed lines to both axes. Then add one shift. If demand rises, draw a second demand curve to the right and mark higher price and quantity. If supply rises, draw supply right and mark lower price and higher quantity.

For a price control, draw equilibrium first. A ceiling below it creates a horizontal legal line, with quantity supplied read from supply and quantity demanded from demand. Their gap is shortage. A floor above equilibrium produces the reverse gap.

A reliable graph begins before the event occurs. Label price on the vertical axis and market quantity on the horizontal axis, draw the original demand and supply curves, and mark their intersection (P_0,Q_0). Only then translate the event into a curve change. This order prevents the common error of comparing a new curve with an equilibrium that was never identified.

Curve shifts come from changes in underlying determinants. Higher consumer income shifts demand right for a normal good. A lower input price shifts supply right. A change in the good’s own price does not shift either relationship. It moves buyers and sellers to different points along their existing curves until the market clears.

When both curves shift, use the direction table rather than guessing. Demand and supply both increasing make quantity rise but leave price ambiguous. Demand increasing while supply decreases makes price rise but leaves quantity ambiguous. “Ambiguous” means the result depends on relative shift sizes, not that economics offers no information.

Follow a two-event chain

Consumers develop a stronger taste for electric bicycles while battery prices fall. Demand shifts right and supply shifts right. Both changes raise equilibrium quantity, so quantity definitely rises. Demand tends to raise price while greater supply tends to lower it, so the price change cannot be known without magnitudes.

Price controls are read from the original equilibrium. A ceiling binds only below equilibrium. At the legal price, trace to demand for quantity demanded and to supply for quantity supplied. The difference is shortage. A floor binds only above equilibrium and creates surplus. The observed amount traded is limited by the short side of the market, although rationing rules may determine who actually trades.

Taxes create a wedge rather than a simple curve label. Mark the buyer price above the seller price by the tax amount and read the common after-tax quantity. Subsidies create the opposite wedge. Legal incidence-who sends money to government-does not determine economic incidence.

After drawing, narrate the mechanism: “Lower production cost shifts supply right, creating surplus at the old price. Sellers compete price downward, and equilibrium quantity rises.” If the graph and sentence disagree, repair the graph. This narration turns a picture into an economic argument.