Joint demand, often associated with complementary goods, describes the interdependent demand for two products that are typically consumed or used together to maximize utility.

• Complementary goods enhance each other’s value when used in tandem; for instance, the demand for cars and petrol is joint, as driving a car requires fuel.
• An increase in the price of one complementary good, such as petrol, leads to a reduction in demand for the other, like cars, reflecting a negative cross elasticity of demand.