A Constant Unitary Elasticity Demand Curve

Constant unitary elasticity means a price change of one percent brings a quantity change of one percent. Using the midpoint method, between A and B the price changes by 66.7% and the quantity demanded also changes by 66.7%, so the elasticity is 1. Between B and C, and between C and D, both changes are again 66.7%.

In absolute terms the price steps are not identical: the price falls by $8 from A to B, by $4 from B to C, and by $2 from C to D, while quantity doubles each time. That is why a demand curve with constant unitary elasticity is steep on the left, flat on the right, and curved overall.