Copyleft is stallman's central legal innovation: using copyright law to guarantee freedom rather than restrict it. Where traditional copyright reserves all rights to the author, copyleft licenses grant broad permissions while attaching a condition — any distributed derivative work must be released under the same terms. The result is a license that is self-propagating: freedom, once granted, cannot be taken away by downstream recipients. stallman has situated copyleft within American political tradition in gpl-and-american-way, connecting the reciprocal obligations it enforces to broader American ideals of liberty.
The name is a deliberate pun on "copyright" — the copyright symbol is rotated and the word reversed to signal that the legal mechanism is being used for the opposite of its conventional purpose. The pun originated in a Symbolics hacker culture context but was adopted by stallman as the name for his legal strategy.
The Mechanism
Copyright gives authors exclusive rights over copying, distribution, and modification. A proprietary software vendor uses these rights to prevent users from sharing or studying the code. Copyleft uses the same rights differently: the author grants permission to copy, distribute, and modify — but only on the condition that all recipients receive the same permissions. Any attempt to distribute a modified version without extending these freedoms to recipients violates the license and therefore infringes copyright.
This is not a separate legal right; it is a condition attached to the exercise of copyright permissions. The legal mechanism is explained in detail under gpl-copyleft-mechanism.
Origin
The concept emerged from a practical problem stallman encountered at the mit-ai-lab in 1980: Symbolics Corporation had taken the MIT AI Lab's Lisp Machine software, improved it under a proprietary license, and was not required to share those improvements. Stallman could see their changes but could not legally incorporate them back. The experience made visceral the problem that copyleft was designed to solve.
The first implementation was the Emacs distribution terms (circa 1984–1985), which required anyone who distributed a modified Emacs to provide the source code. This predates the term "copyleft" and the formal GPL. The gnu-manifesto (1985) articulates the philosophical argument. The gpl-v1 (1989) was the first formal, reusable copyleft license.
Strong and Weak Copyleft
Copyleft exists on a spectrum:
Strong copyleft (the gpl-v2, gpl-v3): Any program that incorporates GPL code and is distributed must itself be GPL-licensed. This is the "viral" property that makes the GPL controversial in commercial contexts.
Weak copyleft (the lgpl): Modifications to the licensed library must be shared, but programs that merely link to the library can be distributed under different terms. Designed for libraries where stallman judged that broader adoption outweighed the strategic cost of permitting proprietary use.
Network copyleft (the agpl): Extends the trigger to include network use, closing the saas-loophole in GPLv2.
The "Viral" Characterization
Critics — particularly in commercial and open source communities — describe copyleft as "viral" because GPL code, once included in a program, requires the entire program to be GPL-licensed if distributed. stallman rejects this framing as a rhetorical attack: the condition only applies when you choose to distribute, and it applies only to the terms of distribution. No one is forced to distribute GPL software.
The viral characterization has had real strategic consequences: many corporations avoided GPL code for years (and some still do), preferring permissive licenses like MIT or Apache. stallman regards this as an intended feature — copyleft is a barrier against appropriation, not a bug.
Copyleft as Legal Hack
stallman and eben-moglen have both described copyleft as a "hack" on copyright — using the legal system against its own normal operation. eben-moglen refined this characterization in his legal scholarship on the GPL. The note copyleft-as-legal-hack explores this framing in depth.
Influence Beyond Software
Copyleft's logic influenced larry-lessig's creative-commons project, particularly the ShareAlike licenses. The free-software-foundation has consistently argued that the ShareAlike principle is sound but that Creative Commons licenses are not appropriate for software. The Wikipedia use of CC BY-SA is a direct descendant of copyleft thinking applied to encyclopedic content.
Copyleft remains the most debated element of stallman's legacy. Its defenders argue it is the only mechanism that prevents freedom from being extinguished by accumulation of proprietary derivatives. Its critics argue that permissive licenses achieve broader adoption and therefore more freedom in practice. stallman regards this as a confusion of freedom with convenience — the strategic core of software-freedom-vs-open-source.