Copyleft

Copyleft is a licensing approach that uses copyright law to guarantee fundamental freedoms: to use, study, modify, and share software and other works. The key mechanism is that redistribution or derivation must be released under the same or compatible copyleft terms, so freedoms persist through subsequent generations. This contrasts with permissive licenses and with traditional copyright that restricts further use.

The concept was developed by the Free Software Foundation in the 1980s as part of the GNU project, led by Richard Stallman. The GNU General Public License (GPL) is the best-known implementation and has inspired many related licenses.

Under a copyleft license, distributing under such terms requires access to the source code and that derivative works carry the same terms. Obligations vary by license and can affect code, libraries, or network use. Some licenses allow combining with proprietary software under certain conditions, while others impose stricter restrictions.

Major forms include strong copyleft (GPL), weak copyleft (LGPL), and network copyleft (AGPL). The Mozilla Public License is another form with file-level copyleft. These choices influence license compatibility with other licenses and with proprietary software, and they shape how software can be integrated in larger projects.

Copyleft has been influential in shaping free and open source software, promoting collaboration and long-term freedom. It also raises concerns about license compatibility, redistribution constraints, and integration with proprietary systems, contributing to ongoing debates in the software community.