Dual licensing
Releasing the same code under two licences: a copyleft licence that is free but infectious, and a commercial licence for anyone unwilling to share their own code.
It requires one party to own the copyright to everything, which is why such projects ask contributors for a CLA — an agreement giving the company the right to relicense the contribution. MySQL built its entire business on the model, as did Qt and early Sleepycat. AGPL is the modern instrument because it closes the network loophole: run the software as a service and the source still has to be released. The criticism is that it isn't genuine community development — contributors donate work to a product that is then sold — and that the licence can be changed whenever the owner wishes, as HashiCorp and Redis demonstrated, triggering the OpenTofu and Valkey forks. Distinct from open core, where different code carries different licences.