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
An increase in the demand for toothpaste results in an increase in the demand for cellular phones.