Complementary Goods are generally taught as a demand-side function. It describes the relationship between two goods that are separate and distinct but often (or always) used together. There is an INVERSE relationship between the change the price of one good and the demand for the other.
Here is an example (source HERE)
You may remember hearing about a shortage of certain meat products at fast-food restaurants in China over the summer, due to the news that a major supplier was handling meat improperly and selling expired meat to restaurants.
The impact reached across oceans, certainly to the fast-food chains like McDonald’s, which changed suppliers and couldn’t keep up with the demand because of the news.
But less meat to eat also means less meat to season, so McCormick & Co. Inc. also lost out. The Baltimore-based spice maker acknowledged the effects in its quarterly report Thursday.
“Our quick service restaurant customers in the Asia-Pacific region are currently being impacted by well-publicized supply issues,” said President and CEO Alan Wilson. “This is affecting our sales results in the region and has us cautious in our near term outlook.”
That region showed a 1 percent decrease in industrial sales as a result.