Two commodities X and Y are in joint supply when they are produced together by the same process, and an increase in the production of one commodity leads to an increase in the production of the other. This means that the production of X and Y are interdependent, and they cannot be produced separately. For example, crude oil and natural gas are in joint supply because they are produced together from the same oil wells, and an increase in the production of crude oil leads to an increase in the production of natural gas as well.