Parks confer many external benefits on society: open space, trees that reduce pollution, and so on. Therefore, the market equilibrium quantity of parks is not equal to the socially optimal quantity. The following graph shows the demand for parks (their private value), the supply of parks (the private cost of producing them), and the social value of parks, including both the private value and external benefits. Use the black point (plus symbol) to indicate the market equilibrium quantity. Next, use the purple point (diamond symbol) to indicate the socially optimal quantity. PRICE OF PARKS Supply (Private Cost) Social Value Demand (Private Value) QUANTITY OF PARKS + Market Equilibrium ◆ Socially Optimal Level