Substitute Goods vs Complementary Goods
Substitute Goods and Complementary Goods are two Supply & Demand concepts in AP Economics that students often mix up. Substitute goods are goods that can be used in place of each other to satisfy a particular need or want. Complementary goods are goods that are typically used or consumed together. Here is how they compare side by side.
When the price of one good increases, the demand for its substitute also increases, as consumers switch to the relatively cheaper alternative. Examples include Coke and Pepsi, or butter and margarine.
When the price of one good increases, the demand for its complement decreases, as consumers buy less of both goods. Examples include cars and gasoline, or printers and ink cartridges.
Substitutes vs Complements: What a Price Change Elsewhere Does
| Substitute goods | Complementary goods | |
|---|---|---|
| Relationship | Used instead of each other | Used together |
| If the price of good A rises | Demand for good B increases | Demand for good B decreases |
| Cross-price elasticity | Positive | Negative |
| Demand curve for B | Shifts right | Shifts left |
| Examples | Coffee and tea, butter and margarine, two airlines on a route | Printers and ink, cars and petrol, phones and cases |
| Effect of more substitutes existing | Demand for the good becomes more elastic | Not directly relevant |
The sign of cross-price elasticity is the whole answer
Cross-price elasticity of demand is the percentage change in the quantity demanded of one good divided by the percentage change in the price of another. If it is positive, the goods are substitutes, because the price of one and the demand for the other move in the same direction. If it is negative they are complements, because they move in opposite directions. If it is approximately zero the goods are unrelated. The magnitude tells you how strong the relationship is, so a cross-price elasticity of 2.4 means very close substitutes and 0.1 means barely related. Questions often give you two percentages and ask for the classification, so compute the ratio, look at the sign, and state the category. Work through it at /calculate/cross-price-elasticity.
Both shift the curve, and neither moves you along it
A change in the price of a related good is a determinant of demand, so it shifts the demand curve for the good you are analysing. It never causes a movement along that curve, because the good's own price has not changed. If the price of coffee rises, the demand curve for tea shifts right, and the price and quantity of tea both rise as the market finds a new equilibrium. Notice the sequence: the shift comes first and the price change is the consequence. Students often shortcut to the conclusion that tea gets more expensive without drawing the shift, and rubrics award the shift. Set it up at /sandbox/supply-demand.
Complements make pricing decisions strange, in a testable way
Because complements are consumed together, a firm selling both can profit by pricing one below cost to sell more of the other. Printers are cheap and ink is expensive. Consoles sell near cost and games carry the margin. That is a real business strategy built directly on negative cross-price elasticity, and it shows up in questions about firms with multiple products. The same logic runs backwards: if a complement becomes more expensive for reasons outside the firm's control, such as a fuel price spike for a car maker, demand for the firm's own good falls even though its own price has not changed. Being able to name the direction and give a reason is what the rubric asks for.
Frequently asked questions
What is the difference between substitutes and complements?
Substitutes are goods used instead of each other, so when the price of one rises, demand for the other increases and cross-price elasticity is positive. Complements are goods used together, so when the price of one rises, demand for the other falls and cross-price elasticity is negative.
How do you tell substitutes from complements using elasticity?
Compute cross-price elasticity: the percentage change in quantity demanded of one good divided by the percentage change in the price of the other. A positive result means substitutes, a negative result means complements, and a value near zero means the goods are unrelated.
Does a change in the price of a substitute shift the demand curve?
Yes. The price of a related good is a determinant of demand, so it shifts the whole curve rather than causing a movement along it. Only a change in the good's own price moves you along its demand curve.
Want the long version? Substitutes vs Complements: How to Tell Them Apart walks through the same comparison as a full guide, with worked examples and the exam traps. This page is the quick side-by-side.
Live Supply and Demand graph. Drag the curves, or open the full version.
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