What is Copyleft?
Copyleft is a licensing concept that requires derivative works to be distributed under the same license as the original work.
Copyleft is a licensing concept that requires derivative works to be distributed under the same license as the original work. It ensures that software remains free/open and prevents proprietary forks.
Strength spectrum: Strong copyleft (GPL — any "derivative work" must be GPL, including applications that link to GPL code), Weak copyleft (LGPL — only modifications to the library itself must be shared, not the application using it), File-level copyleft (MPL — changes to MPL files must be shared, but files in the rest of the project don't), and Network copyleft (AGPL — extends copyleft to software accessed over a network, closing the "SaaS loophole").
The AGPL is particularly important for SaaS companies: regular GPL only requires source disclosure when distributing binaries. AGPL extends this to network access — meaning hosting GPL'd code as a web service triggers the source-sharing requirement.
Why It Matters
Copyleft prevents companies from taking open-source code, making improvements, and keeping those improvements proprietary. It ensures the commons stays open. For commercial software, copyleft dependencies can force unwanted source disclosure.
Frequently Asked Questions
What is copyleft?
A license requirement that derivative works must use the same license. Ensures software stays open. GPL is the most famous copyleft license — if you modify GPL code, your modifications must also be GPL.
Is copyleft good or bad?
Depends on your perspective. For community: copyleft ensures improvements stay open (good). For commercial software: copyleft can force source disclosure (risky). Many companies have policies prohibiting copyleft dependencies.
Related Terms
Need Expert Help?
Richard Ewing is a Product Economist and AI Capital Auditor. He helps companies translate technical complexity into financial clarity.
Book Advisory Call →