A price floor is a government- or group-imposed price control or limit on how low a price can be charged for a product, good, commodity, or service. A price floor must be higher than the equilibrium price in order to be effective. The equilibrium price, commonly called the "market price", is the price where economic forces such as supply and demand are balanced and in the absence of external influences the (equilibrium) values of economic variables will not change, often described as the point at which quantity demanded and quantity supplied are equal (in a perfectly competitive market). Governments use price floors to keep certain prices from going too low. Two common price floors are minimum wage laws and supply management in Canadian agriculture. Other price floors include regulated US airfares prior to 1978 and minimum price per-drink laws for alcohol. While price floors are often imposed by governments, there are also price floors which are implemented by non-governmental organizations such as companies, such as the practice of resale price maintenance. With resale price maintenance, a manufacturer and its distributors agree that the distributors will sell the manufacturer's product at certain prices (resale price maintenance), at or above a price floor (minimum resale price maintenance) or at or below a price ceiling (maximum resale price maintenance). A related government- or group-imposed intervention, which is also a price control, is the price ceiling; it sets the maximum price that can legally be charged for a good or service, with a common government-imposed example being rent control. Effectiveness A price floor could be set below the free-market equilibrium price. In the first graph at right, the dashed green line represents a price floor set below the free-market price. In this case, the floor has no practical effect. The government has mandated a minimum price, but the market already bears and is using a higher price. By contrast, in the second graph, the dashed green line represents a price floor set above the free-market price. In this case, the price floor has a measurable impact on the market. It ensures prices stay high, causing a surplus in the market. Effect on the market A price floor set above the market equilibrium price has several side-effects. Consumers find they must now pay a higher price for the same product. As a result, they reduce their purchases, switch to substitutes (e.g., from butter to margarine)