Stallman used copyright law to make software stay free.

In the 1980s, Richard Stallman's GNU Project was writing a free operating system while software was increasingly locked behind proprietary licenses. Putting GNU software in the public domain would have let anyone take it and close it up again.

So the project copylefted it. The Free Software Foundation explains that copyleft says anyone who redistributes the software, with or without changes, must pass along the freedom to further copy and change it. The GNU General Public License, version 2 of June 1991, is the legal instrument that makes that condition stick.

The mechanism uses copyright law against itself: the license grants rights, and the grant is conditional on derivative works being licensed the same way. Legal scholar Eben Moglen's 1999 essay described the GPL as using copyright to counterfeit the phenomena of anarchism, a private property right deployed to guarantee a commons.

Use copyright to force sharing, not forbid it

It needs no central enforcement: the condition rides inside every copy.

Derivative works must give back, so improvements flow to the community.

Copyright law, the tool of enclosure, becomes the tool of openness.

Companies can still use and sell the software; they just cannot close it.

Attach the obligation to the thing people want: instead of asking for generosity, make the license condition travel with every copy and every improvement.

The GPL became the license of Linux, GNU tools and thousands of projects, and copyleft spread into art, hardware and data licenses. It remains the reference point for every open-source license debate.