Mathematically, the variable representing the price of the complementary good would have a negative coefficient in the demand function. For example, Qd = a - P - Pg where Q is the quantity of automobiles demanded, P is the price of automobiles and Pg is the price of gasoline. The other main category of related goods are substitutes. Substitutes are goods that can be used in place of the primary good. The mathematical relationship between the price of the substitute and the demand for the good in question is positive. If the price of the substitute goes down the demand for the good in question goes down.