Copyleft
License strategy: requires derivatives to be distributed under the same license. Ensures free software stays free. GPL is the best-known copyleft license.
Richard Stallman coined the concept in the 1980s. Symbol: mirrored ©. Strong copyleft (GPL, AGPL) requires full source release on distribution. Weak copyleft (LGPL, MPL) allows linking from proprietary code. Companies often avoid GPL due to "viral" effect — Apple replaced bash → zsh in macOS to escape GPLv3. AGPL closes the SaaS loophole (must release source even if just hosting the service). Permissive licenses (MIT, BSD, Apache) are the counterpole.