Examples of substitute goods include margarine and butter, tea and coffee. Substitute goods not only occur on the consumer side of the market but also the producer side. Substitutable producer goods would include: petroleum and natural gas (used for heating or electricity). The degree to which a good has a perfect substitute depends on how specifically the good is defined. Take for example, the demand for Rice Krispies cereal, which is a very narrowly defined good as compared to the demand for cereal generally. The fact that one good is substitutable for another has immediate economic consequences: insofar as one good can be substituted for another, the demands for the two kinds of good will be interrelated by the fact that customers can trade off one good for the other if it becomes advantageous to do so.