A method and a system for making contextual recommendations to users on a network-based system. For example, a first and second related category are selected and a relationship score is calculated representing an aspect of the relationship between the first and second related categories. A relationship type is selected to represent the relationship between the first and second categories using the relationship score. The relationship type is one of either a complement or a substitute. A complement relationship indicates a complementary relationship between the first and second related categories. While a substitute relationship indicates a substitutionary relationship between the first and second categories. The relationship type is stored associated with the first and second related categories. Using the selected relationship type, a recommendation is provided to a user on the network-based system.