The term complementary goods refers to goods or products that complement each other in their common use. In this context, the use of one good (product A) necessarily presupposes the use of the other good (product B). An example of a complementary relationship is the fountain pen and ink.
The price policy effects of complementary goods have negative cross-price elasticity. This means that a price change (e.g. price reduction) of one good (product A) not only leads to a (in this case positive) change in sales of this good, but also - in the same direction - of the complementary good (product B), without a price change being present here.