In economics, the demand curve for a perfectly inelastic good is vertical.
Perfectly inelastic demand means the quantity demanded remains unchanged when price changes. On a graph with price on the vertical axis and quantity on the horizontal axis, every observed price is paired with the same quantity, producing a vertical line. Its price elasticity of demand is zero.
The phrase describes an idealized limiting case. Real products rarely have perfectly inelastic demand across all prices and time periods. In the short run, however, some necessities may have very low elasticity because consumers cannot quickly find substitutes or change their habits. Life-saving medicines are often discussed as an example, although their actual responsiveness varies by patient, insurance, alternatives, and regulation.
Perfectly inelastic demand differs from perfectly elastic demand, which is represented by a horizontal line and is associated with an unlimited response to tiny price differences at a given market price.