An important source of supply and demand changes are changes in the markets of complements. A decrease in the price of a demand-complement increases the demand for a product, and similarly, an increase in the price of a demand-substitute increases demand for a product. This gives two mechanisms to trace through effects from external markets to a particular market via the linkage of demand substitutes or complements. For example, when the price of gasoline falls, the demand for automobiles (a complement) overall should increase. As the price of automobiles rises, the demand for bicycles (a substitute.
