Price

In order to measure the ebb and flow of supply and demand, a measurable value is needed. The oldest and most commonly used is price, or the going rate of exchange between buyers and sellers in a market. Price theory, therefore, charts the movement of measurable quantities over time, and the relationship between price and other measurable variables. In Adam Smith's Wealth of Nations, this was the trade-off between price and convenience.A great deal of economic theory is based around prices and the theory of supply and demand. In economic theory, the most efficient form of communication comes about when changes to an economy occur through price, such as when an increase in supply leads to a lower price, or an increase in demand leads to a higher price.

Exchange rates are determined by the relative supply and demand of different currencies an important issue in international trade.In many practical economic models, some form of "price stickiness" is incorporated to model the fact that prices do not move fluidly in many markets. Economic policy often revolves around arguments about the cause of "economic friction", or price stickiness, and which is, therefore, preventing the supply and demand from reaching equilibrium.