Definition: Goods A and B are complements if as the price of good A falls (rises), the demand for good B increases (decreases).
Example: Automobiles and gasoline
Evidence: From the Wall Street Journal
A Deeper Story: While gas and automobiles are complements in general, the price of gas also changes the types of cars bought. As the price of gas decreases, consumers are likely to buy less fuel efficient cars. Comparing the period January 1, 2014-October 31, 2014 with January 1, 2015-October 31, 2015, the sale of 'Cars' decreased by 1.6%. The sale of 'Light Duty Trucks' increased by 12.7% with the sale of 'SUV's' increasing by 11% and the sale of 'Cross-over SUVs' increasing by 17.9%.