a natural graphical representation. Demand curves were first drawn by Augustin Cournot in his Recherches sur les Principes Mathématiques de la Théorie des Richesses (1838) – see Cournot competition. Supply curves were added by Fleeming Jenkin in The Graphical Representation of the Laws of Supply and Demand... of 1870. Both sorts of curve were popularised by Alfred Marshall who, in his Principles of Economics (1890), chose to represent price – normally the independent variable – by the vertical axis; a practice which remains common. If supply or demand is a function of other variables besides price, it may be represented by a family of curves (with a change in the other variables constituting a shift between curves) or by a surface in a higher dimensional space. Microeconomics Equilibrium Generally speaking, an equilibrium is defined to be the price-quantity pair where the quantity demanded is equal to the quantity supplied. It is represented by the intersection of the demand and supply curves. The analysis of various equilibria is a fundamental aspect of microeconomics: Market equilibrium: A situation in a market when the price is such that the quantity demanded by consumers is correctly balanced by the quantity that firms wish to supply. In this situation, the market clears. Changes in market equilibrium: Practical uses of supply and demand analysis often center on the different variables that change equilibrium price and quantity, represented as shifts in the respective curves. Comparative statics of such a shift traces the effects from the initial equilibrium to the new equilibrium. Demand curve shifts: When consumers increase the quantity demanded at a given price, it is referred to as an increase in demand. Increased demand can be represented on the graph as the curve being shifted to the right. At each price point, a greater quantity is demanded, as from the initial curve to the new curve . In the diagram, this raises the equilibrium price from to the higher . This raises the equilibrium quantity from to the higher . (A movement along the curve is described as a "change in the quantity demanded" to distinguish it from a "change in demand," that is, a shift of the curve.) The increase in demand has caused an increase in (equilibrium) quantity. The increase in demand could come from changing tastes and fashions, incomes, price changes in complementary and substitute goods, market expectations, and number of buyers. This