Markets: institutions that brings buyers and sellers together. Price system: is a name given to the market economy because prices provide considerable information to both buyers and sellers. Buyer is the demand side, seller is the supply side of the market. The maximum amount of a product that buyers are willing to purchase over some period at various prices. For every p there is a q ( change of p, there is a change of q) How does change of price affect the demand . Price change equals the quantity change, not the demand. Horizontal summation of all individual demand curves. Horizontal summation: adding the number of units of products that will supplied and purchased of each price. Non price factors affect demand; they are held constant for drawing a demand curve. Tastes and preferences, income, prices of related goods- substitutes and complements, number of buyers, expectations about future prices.