Panel (a), a price ceiling. Left to the market, a new back-pain drug sells at $600 a month to 20,000 people. Consumer surplus is T + U and producer surplus is V + W + X. A price ceiling of $400 makes firms produce only 15,000. Two things happen. The total surplus falls: the loss, U + W, is the deadweight loss, money thrown away that benefits no one, because the control blocks trades both sides were willing to make. And some producer surplus moves to consumers: the new consumer surplus is T + V and the new producer surplus is X, so V is transferred. The consumers' gain is less than the producers' loss, which is the deadweight loss seen another way.
Panel (b), a price floor. Movie tickets sell at $8 with 1,800 attending. Consumer surplus is G + H + J, producer surplus I + K. A price floor of $12 cuts the quantity demanded to 1,400. The new consumer surplus is G and the new producer surplus is H + I: H is transferred from consumers to producers, and J + K is the deadweight loss.
A ceiling, like rent control, transfers producer surplus to consumers, which is why consumers often favor them; a floor, like a farm price support, transfers consumer surplus to producers. Both block trades and create deadweight loss. Move the sliders to change the controls; turn on the shading to see the areas.