The cross-price elasticity of demand measures how sensitive the demand curve is to changes in the price of some other good or service by measuring how far the demand curve shifts along the quantity axis as the other price changes. More precisely it measures the percentage change in quantity demand per percentage change in the other goods price. Using the general demand curve given above in Equation (6A-5) we get:

Again, analyzing Equation 6A-5 reveals that when the price of corn syrup increases by $1 (i.e., ΔP_{corn syrup} = 1), then the quantity of maple syrup demanded increases by . Therefore, the ratio is equal to .

Since b, P_{corn syrup}, and Q^d are all positive, a₁ determines whether the cross-price elasticity of demand is positive or negative:

If

If

So if a₁ > 0, the other good is a substitute (as corn syrup is for maple syrup) and an increase in the price of the second good causes an increase in demand for the first good. Similarly, if a₁ < 0, the other good is a complement (as pancake mix is for maple syrup) and an increase in the price of the second good causes a decrease in demand for the first good.