Copyleft licences: the freedom that can't be taken back

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This English version was produced by automatic AI translation. Only the French version is reviewed by the author, so it remains the authoritative reference. Spotted a mistake or an awkward turn of phrase? Please report it by opening a pull request — contributions are welcome.

The first instalment of this series explored permissive licences (BSD, MIT, Apache) through FreeBSD: they leave everyone free to do what they like with the code, including closing it back up. Here is their counterpart: copyleft licences.

The difference comes down to a single idea. Where permissive licences allow closing the code, copyleft requires that any redistributed version stay free, source code included. The best known is the GPL, with its LGPL and AGPL variants. Its purpose is not merely to share code: it is to ensure that freedom cannot be taken back from the user downstream.

What is copyleft?#

Copyleft is a reversal of copyright. Rather than giving up their rights (as with a dedication to the public domain) or using them to forbid (as with proprietary software), the author uses them to guarantee one thing: any redistributed version — including a modified one — must remain under the same free conditions, source code included. Freedom thus becomes transitive, forced to “trickle down” to the last link in the chain; no one can close it up downstream. This is the exact opposite of permissive licences, which specifically allow closing the code.

The word itself is a pun, older than the GPL: it appears as early as 1976 in the distribution notice of Li-Chen Wang’s Palo Alto Tiny BASIC — “@COPYLEFT — ALL WRONGS RESERVED,” a nod to copyright’s “All rights reserved.” But at the time it was only a joke, with no legal mechanism. It was Richard Stallman who turned it into a genuine licensing strategy — he recounts adopting the term after receiving an envelope bearing the note “Copyleft — all rights reversed.”

Sources: “What is copyleft?” (GNU) · “All rights reversed” and the origin of the term (Wikipedia).

How the GPL came to be#

The copyleft mechanism did not appear all at once. Its first incarnation is the GNU Emacs General Public License, around 1985 — born in part from a mishap: code that Stallman had used (the Gosling Emacs editor) was bought up and then closed by a third-party vendor, and he wanted to prevent that from happening again. Analogous licences followed for the GCC compiler and the GDB debugger. The problem: each was specific to its program (you had to copy the text and name the software concerned), so they were mutually incompatible.

Hence the decisive step: in 1989 (the name “GNU General Public License” appears as early as June 1988), the GPL version 1 generalised this text. That is the whole meaning of “general” here: a single licence text, independent of any program, applicable as-is to any software — instead of a licence to be copied and adapted for each project. The GPL therefore did not invent copyleft (both the term and the mechanism predated it), but it was the first reusable licence to use it — and that is what drove its massive adoption.

The rest comes down to two dates. GPLv2 (1991) adds the so-called “Liberty or Death” clause (in essence: if an external constraint — a patent, for example — prevents you from distributing while respecting the freedoms, you may not distribute at all); it is the one that became the reference licence — the Linux kernel is under GPLv2. GPLv3 (2007) finally modernises the text against more recent threats (patents, tivoization), detailed further down.

Sources: History of the GNU GPL (free-soft.org) · GNU General Public License (Wikipedia) · Sam Williams, Free as in Freedom, ch. 9 (O’Reilly).

Two axes to keep from getting lost#

The names — GPL, LGPL, AGPL, and their “v2”/“v3” — mix two independent dimensions. Keeping them distinct avoids a great deal of confusion:

  • The strength of the copyleft — how far the obligation to stay free extends: LGPL (weak) → GPL (standard) → AGPL (network, the most far-reaching).
  • The version over time — v1 (1989) → v2 (1991) → v3 (2007): above all a legal modernisation.

A name like “AGPLv3” is thus merely a point on each of these two axes. The next two sections take them one at a time.

The GPL family: LGPL, GPL, AGPL (the “strength” axis)#

  • GPL — the standard copyleft. If you distribute a program that includes GPL code, the whole thing must be published under the GPL, sources included.
  • LGPL — a weak copyleft, designed for libraries. Proprietary software can link to an LGPL library without itself having to become free; only modifications to the library stay copyleft. Born in 1991 under the name Library GPL, it was renamed Lesser GPL (v2.1) in 1999 — a name change that reflects the FSF’s intent: not to make it the default choice.
  • AGPL — a “network” copyleft, the most far-reaching. It closes the “SaaS loophole”: with the GPL, running a modified program as an online service (without distributing the binary) does not trigger the obligation to share the sources. The AGPL extends it to users who interact with the program over a network (its “Remote Network Interaction” clause). It descends from the Affero GPL (2002); the GNU AGPLv3 dates from November 2007.

Sources: GNU LGPL (gnu.org) · History of the LGPL (FOSSA) · GNU Affero GPL (Wikipedia).

That leaves the other axis: the evolution of the versions. This is where GPLv3 comes in, whose most emblematic case — the one through which the subject is most often (mis)understood — is tivoization.

Tivoization and the asymmetry of power#

Let us pause on a key concept, because it is often poorly explained: tivoization.

The term comes from the TiVo box, a video recorder that used… Linux, under the GPLv2 licence. In keeping with the GPL, TiVo did publish the source code. But the device was designed to run only binaries signed by the manufacturer: the user could read the code, modify it, recompile it… without ever being able to run their modified version on their own box. Freedom on paper, the lock in the hardware. Richard Stallman coined the word to name this workaround, and the FSF explicitly banned it in GPLv3 (2007) through a so-called “anti-tivoization” clause.

A clarification is needed here, because the shortcut is tempting: you might think tivoization has to do with permissive licences — it’s the opposite. Tivoization is a copyleft problem. If TiVo had to play tricks with the hardware, it is precisely because GPLv2 required it to hand over the source code: unable to hide the sources, it locked the hardware to prevent modifications anyway. With a permissive licence, that detour is pointless: the manufacturer has no need to lock anything, since it can simply never publish the code at all. Permissive is even more radical — it makes the lock legally free of charge — but that has nothing to do with tivoization.

A revealing detail: the Linux kernel stayed on GPLv2 and never moved to GPLv3. Linus Torvalds explicitly rejected the anti-tivoization clause, which he considers outside the scope of a software licence. So the topic divides opinion right to the heart of the free-software world — so much so that I devote a separate article to it.

What remains is the essential point: the asymmetry of power. Whether through tivoization (locked copyleft) or through outright closing (permissive), the result for the user is the same. The model benefits mega-corporations enormously — total freedom to innovate on a free base, massively reduced R&D costs (decades of OS development inherited without having to build it), industrial secrets perfectly protected. But for users, SMEs and individuals, it is the exact opposite: impossible to repair, obsolescence engineered by a mere software update, total dependence on the manufacturer. The console you paid for is one you do not control.

Sources: Tivoization (Wikipedia) · Richard Stallman, “Why Upgrade to GPLv3” (GNU) · “Tivoization & Your Right to Install” (Software Freedom Conservancy)

Beyond tivoization: the other contributions of GPLv3#

Tivoization is the best-known addition in v3, but not the only one. The others are aimed above all at plugging legal loopholes that had appeared since 1991:

  • Patents — v3 includes an explicit patent grant: whoever distributes GPLv3 code grants recipients a licence to their own patents covering that code, which protects them from a later attack.
  • Compatibility with Apache 2.0 — a direct consequence: Apache 2.0’s patent clause was an “additional restriction” incompatible with GPLv2; GPLv3 makes it compatible, which finally allows the two code bases to be combined.
  • Anti-DRM — v3 specifies that GPL software cannot serve as an “anti-circumvention” lock enforceable against the user.
  • Internationalised wording — legal vocabulary less centred on US law, so as to hold up better outside the United States.

Sources: “A Quick Guide to GPLv3” (GNU) · Stallman, “Why Upgrade to GPLv3” (GNU) · Apache Software Foundation, “Apache License v2.0 and GPL Compatibility”.

Two philosophies, one wish#

At bottom, permissive and copyleft do not oppose each other on the technical level: they carry two philosophies. Permissive licences are in the spirit of open source — making the best possible software, the most widely reused, while leaving everyone free to do what they like with it, including closing it up. Copyleft is in the spirit of free software in the FSF’s sense — ensuring that the user’s freedom cannot be taken away from them along the way: there it is transitive, forced to “trickle down” to the last link. The first approach is primarily pragmatic; the second, primarily ethical.

Source: Richard Stallman, “Why Open Source Misses the Point of Free Software”.

I draw no lesson to hand out. As I recount in the founding article, I myself earn my living on proprietary software — which is to say I would be poorly placed to tell others what they ought to do. My preference goes to the philosophy that places the user’s freedom at the centre; but it is a preference, not a verdict.

So I will not end with a judgement, but with a wish: for a computing world that is more free, where everyone can understand, repair and extend the tools their life depends on. Permissive and copyleft both contribute to it, each in its own way — and FreeBSD, whose code runs part of the world’s infrastructure, is fully a part of it.

Sources and references#

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By Yanal-Yves FARGIALLA • Updated on July 4, 2026 (AI-assisted writing, final review by the author)
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