Final answer:
Two goods are complements if an increase in demand for one leads to an increase in demand for the other, as their use is interconnected. Complementary goods like golf clubs and golf balls, or bread and peanut butter, enhance each other's consumption. This contrasts with substitute goods, where a decrease in price of one reduces demand for the other.
Step-by-step explanation:
Two goods are complements if an increase in demand for the first good leads to an increase in demand for the second good. This is because the use of one good tends to enhance the consumption of the other. For instance, if there is an increase in demand for golf clubs, this typically leads to an increase in demand for golf balls, as they are complementary goods. Similarly, a decrease in the price of one complement, like bread, can lead to an increase in the quantity demanded of its complement, like peanut butter.
It's important to note that the opposite effect occurs when two goods are substitutes. In that case, a decrease in the price of one good can lead to a decrease in the quantity demanded of the substitute good, such as when cheaper plane tickets result in fewer train tickets sold.