What is Complementary goods?
Also known as: complements
Goods that are consumed together, where one does not give full benefit without the other. Their sign is a negative cross elasticity of demand: if one good becomes cheaper, demand for the other rises, and if it becomes more expensive, demand falls. Cross elasticity = percentage change in demand for the second good ÷ percentage change in the price of the first good. A positive coefficient indicates substitute goods, a coefficient close to zero indicates unrelated goods. The larger the negative number in absolute value, the stronger the relationship. The relationship is not always symmetric and should be checked with market data, not guesswork. The business takeaway: a discount on one good often increases sales of the other.
Source: Economics Online: complements