Copyleft is the most consequential single idea in software licensing, and it fits in one sentence: whatever you distribute, you must pass on under the same terms you received. It is copyright law turned against its usual reflex. Instead of using the author's exclusive rights to restrict the code, the license uses them to guarantee that the code stays free for everyone downstream, forever.

The promise sounds simple. The machinery underneath it is not, and the arguments about where copyleft should reach have shaped twenty years of license drafting.

The reciprocity at the center

Richard Stallman framed copyleft in the 1980s as a jujitsu move: the law grants the author exclusive control, so the author writes a license that uses that control to forbid privatization. If you distribute a copy of a copylefted program, modified or not, you must give your recipients the same freedoms you got, including the source code and the right to pass it on again. The clause is recursive. Each new holder is bound to the next.

That recursion is why copyleft projects accumulate a protective commons. Linux, the GNU toolchain, and decades of embedded code are covered by this one mechanism. The same reciprocity instinct shows up far outside software: the question of carrying credit and conditions forward is just as live in an independent image practice, where every reuse of a picture inherits the obligations of the original license.

The mechanism only works because the author keeps the copyright. Copyleft is not the absence of ownership; it is ownership deliberately aimed at openness. The license is the instrument through which the holder's exclusive rights are spent, once, to buy a commons that cannot be bought back.

Strong and weak are a spectrum

Not all copyleft pulls with the same force. The GPL is strong copyleft: combine the code into a larger work and distribute it, and the whole work must follow. The Lesser GPL weakens the trigger so libraries can be linked into proprietary programs while the library itself stays free. The Mozilla Public License operates at the level of files: changes to a covered file stay open, but new files you write beside them can remain closed.

Choosing where on that spectrum to sit is a design decision about how far the commons should spread. The stronger the copyleft, the larger the zone of code that must stay open around it; the weaker the copyleft, the easier the code is to combine with proprietary work. There is no correct answer, only a choice about what the license is for, and the binder files each text by where it plants its flag on that line.

The network hole and the Affero answer

The Affero clause is the point where copyleft stops being about shipped software and starts being about the code's behavior wherever it runs. It is the answer the open source shelf gave to the cloud before the cloud was named, and it is the reason the later source-available texts had to reach past distribution to find a different trigger.

Classic copyleft triggered on distribution, and the rise of software delivered as a service exposed the gap. A company could run GPL code for millions of users, modify it deeply, and never distribute anything, so the share-alike clause never fired. The Affero GPL closed that hole by treating interaction over a network as a triggering event: offer the program as a service and you must offer your source to its users.

This is the clause the cloud era eventually answered with the Server Side Public License, which reached further and stepped off the open source shelf entirely.

What enforcement looks like

Copyleft only exists if someone can enforce it, and enforcement has a culture of its own. Most violations are settled quietly: a device maker ships GPL firmware without source, a polite letter arrives, the code appears. Organizations like the Software Freedom Conservancy and gpl-violations built their work on patient compliance rather than lawsuits. The rare litigated cases, including the long-running dispute over GPL components in VMware products, remind the shelf that the clause is a legal instrument, not a slogan.

Why it remains contested

Critics call copyleft restrictive, because it limits what a downstream developer may do with the code, namely closing it. Defenders answer that the restriction exists precisely to protect everyone else's freedom. Both statements are true at once, which is why the argument never ends. The permissive licenses answer the same problem with a shrug: take the code, keep the notice, do what you like.

The promise kept

What copyleft actually promises is narrower and more durable than its reputation. It does not promise that software will be free of charge, or that its authors will be paid, or that the commons will be used wisely. It promises that whatever commons exists will not be quietly fenced off downstream. In a binder full of texts that trade on the word open, that is the clause that means it, and the reason the GPL has outlasted every attempt to draft around it.

What the text grants

All four freedoms plus a recursive duty: pass the code on under the same terms, source included.

What a reader should check

Whether the copyleft is strong, weak, or file-level, and whether it reaches software offered over a network.

The limit of this reading

The clause binds only on distribution or network offer; private modification behind closed doors stays private.