Demand levels for some categories of products are closely interdependent. When one type of a product can serve as a complement for another product, the levels of demand for these goods are positively related. An increase in the demand for one product usually increases the demand for its complementary product. Similarly, a decrease in the demand for one good causes a decrease in the demand for its complementary product.
A decrease in the demand for CD players results in a decrease in the demand for CD's provides an example of the relationship between product demands described above.