Copyleft
Noun · Adjective · Open Source · Origin: 1976
Definitions
A licensing philosophy that uses copyright law to require that modified versions of a work must also be distributed under the same terms. A deliberate inversion of 'copyright' — using the legal system to guarantee freedom rather than restrict it.
In plain English: A type of license that says: this code is free, and anything built from it must stay free too — you can't take open-source code and make it proprietary.
The 'viral' nature of copyleft is its most powerful and controversial feature. If you incorporate GPL code into your project, your entire project must be released under GPL. This is by design — Richard Stallman's goal was to ensure software freedom propagates, not just exists.
Example: 'We can't use that library — it's GPL and our product is proprietary. Using it would legally require us to open-source our entire codebase.'
Source: viral mechanism
Etymology
- 1976
- Li-Chen Wang marks his Palo Alto Tiny BASIC with '@COPYLEFT ALL WRONGS RESERVED' — the first known use of the term
- 1985
- Richard Stallman formalizes copyleft as a legal mechanism in the GNU project: you can copy and modify, but must preserve the same freedom
- 2000s
- Copyleft vs. permissive becomes the defining philosophical divide in open source, with both camps thriving
Origin Story
Turning copyright law inside out
In 1985, Richard Stallman faced a paradox: he wanted software to be free forever, but existing copyright law made it trivial for someone to take free code, modify it, and release the modified version as proprietary software. Copyright protected the rights of restricters, not sharers.
Stallman's brilliant hack was to use copyright law against itself. The GNU General Public License (GPL) used copyright to require that any derivative work must also be distributed under the same free terms. He called this 'copyleft' — a playful inversion of 'copyright,' sometimes represented by a reversed C in a circle.
Copyleft was legally controversial and philosophically divisive. Some saw it as a guarantee of freedom; others called it 'viral' because it 'infected' any code it touched. The tension between copyleft (GPL) and permissive licenses (MIT, BSD) remains one of open source's most enduring debates.
Coined by: Richard Stallman
Context: GNU Project, 1985
Fun fact: The copyleft symbol — a reversed letter C in a circle — has no legal standing but has become an internationally recognized symbol of the free software movement.