Shape of Demand Curves and Elasticity

The price elasticity of demand is closely related to the slope of the demand curve but the slope of a demand curve is different from its price elasticity of demand. However, there are exceptions in which the price elasticity of demand can be understood by looking at the slope of the demand curve. These cases are: (i) when price and quantity are identical, by looking at the slopes of the two intersecting demand curves it can be said which one is more elastic than the other. (ii) If the demand curve is vertical, its slope and its price elasticity is zero, and (iii) If the demand curve is horizontal, its slope and the price elasticity would be infinite.