Related goods are of two type: Substitute goods and Complementary goods.
(i) In case of substitute goods: Increase in the price of one good results in increase in the demand of its substitute good Similarly Tea and Coffee. When the price of tea increases/ then the consumers will starts consuming more of coffee in place of tea, as a result, demand of coffee increases.
(ii) In case of complementary goods: When price of one commodity increases demand for its complementary good falls. For example: increase in price of petrol will decrease the demand for car.
Following diagram shows this:
