Consumers regard dell computers and apple computers as substitutes. If the price of a dell computer decreases, then the demand for the apple computers would decrease and the demand for dell computers would increase. Substitute goods are goods which are seen by the consumer as similar or the same. As a general rule in economics, products A and B are substitute goods, if and only if the demand of A increases as the price of B increases. In other words, there is a positive elasticity of demand between the goods. Other examples are Pepsi and Coke, tea and coffee, butter and margarine.
