Definition:
Copyleft is a licensing method that grants broad permission to use, copy, modify and redistribute a work, provided that distributed derivative versions preserve those freedoms under the same or a compatible licence.
The mechanism relies on copyright: the rights holder retains those rights and uses a licence to establish permissions and obligations. Copyleft is therefore neither the public domain nor an absence of copyright. It also does not necessarily mean that a work must be provided at no cost.
How copyleft works
A copyleft licence allows specified uses without requiring separate permission each time. In return, anyone distributing copies or modified versions must follow the licence conditions, retain the relevant notices and, where applicable, provide the source code or editable form of the work.
The requirement to share under the same terms is usually triggered when the work or a derivative version is distributed. Modifying a program for private use does not in itself require its publication, although some licences contain specific conditions for software used over a network. The exact scope always depends on the text and version of the applicable licence.
Origin of copyleft
The term plays on the English words copy and left in response to copyright. Its common symbol is a reversed C inside a circle, but the symbol is informal and does not replace a licence notice or the licence text.
Ideas associated with copyleft emerged in software communities during the 1970s and 1980s. Richard Stallman and the GNU Project systematised them within the free software movement. The GNU General Public License (GPL), first published in 1989, became the best-known example.
Forms of copyleft
The degree of reciprocity is not identical across licences. The main forms differ according to how much of a derivative work must preserve the same freedoms:
- Strong copyleft: Requires a distributed derivative work to use the same licence. The GNU GPL is the best-known software example.
- Limited or weak copyleft: Restricts reciprocity to particular files, libraries or components. The GNU LGPL and Mozilla Public License apply this approach with different scopes.
- Network copyleft: Adds a requirement to offer the corresponding source code to people interacting with a modified version over a network. The GNU AGPL uses this model.
Outside software, the Creative Commons ShareAlike conditions in CC BY-SA and CC BY-NC-SA require adaptations to be shared under the same or a compatible licence. Not every Creative Commons licence includes this condition, and the organisation itself recommends software-specific licences for software.
Differences from other models
Copyleft is not the legal opposite of copyright. It is a way of using copyright to preserve permissions for future users. It also differs from other models with which it shares some features:
- Copyright: Is the legal framework that grants exclusive rights to an author or rights holder. A copyleft licence operates within that framework.
- Public domain: Allows a work to be reused without the conditions of a copyleft licence, although particular rights may continue to apply in some jurisdictions.
- Permissive licences: Authorise use, modification and redistribution with fewer obligations and allow a derivative work to be distributed under other terms, including proprietary ones.
- Open source: Includes both copyleft and permissive open-source licences. Not all open-source software therefore uses copyleft.
Copyleft in digital projects
In a digital project, the particular licence determines which notices need to be retained, when source code must be provided and the terms under which a modification can be redistributed. Licence compatibility is especially important when libraries, code snippets, documentation and other resources are combined in a single product.
Copyleft does not in itself prevent commercial use. A program or work may be sold, supported through paid services or used within a commercial activity, provided that its distribution follows the permissions and obligations established by its licence.