Late in 2016, Airbnb, the platform that disrupted the hospitality industry by matching travelers with short-stay accommodations in private homes, announced that it was expanding its travel offerings to include restaurant reservations, audio walking tours – perhaps even car rental reservations.

Airbnb’s expansion of its platform’s wildly successful formula into related travel services was part of a process that platform experts call complementary growth – taking a successful platform idea and branching into related businesses. Annabelle Gawer, co-director of the Centre for the Digital Economy at the University of Surrey in Guildford, England, said that after the huge investment needed to start a platform ecosystem, it makes good business sense to expand.

Once you have an installed base and you want to keep adding value to it and retain the engagement of people, you can do that by adding different sides to your platforms by bringing in new groups to interact with your existing side,” Gawer said, referring to each group of users or producers on a platform as a “side.”