Complementary products (we refer to products 1 and 2)
These are products which are usually used together.
Products 1 and 2 are complementary products, if the growth in demand for one of them results in an increase in demand for the other, and vice-versa – if a decrease in demand for one product results in a reduction in demand for the other.
In general: the demand for them at any time is in the same direction.
Examples of complementary products: vehicles and gas, computers and software.