Quantity demanded: the amount of a good or service that consumers want to purchase during some time period. A basic economic hypothesis is that the price of product and the quantity demanded are related negatively, other things being equal. That is, the lower the price, the higher the quantity demanded; the higher the price, the lower the quantity demanded. Consumers" income, prices of other goods, tastes, population, Normal good: when the quantity demanded increases when income rises. Inferior good: when the quantity demanded falls when income rises. Substitutes in consumption: goods that can be used in place of another good to satisfy similar needs or desires. Complements in consumption: goods that tend to be consumed together. Quantity supplied: the amount of a commodity that producers want to sell during some time period. Supply: the entire relationship between the quantity of some commodity that producers wish to sell and the price of that commodity, other things being equal.