Computers and software are complementary goods. When consumers buy a computer, they often need software to use it effectively. Similarly, when consumers purchase software, they need a computer to run it. An increase in the demand for computers would likely lead to an increase in the demand for software,
Cars and gasoline are demanded together as part of a single final product: transportation. When consumers buy cars, they also need gasoline to fuel them. Likewise, the demand for gasoline is closely tied to the number of cars on the road. As the demand for cars increases, the demand for gasoline to power those cars also increases. Therefore, cars and gasoline exhibit joint demand.