What is a Copyleft License? A UK Guide

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Ever downloaded a free piece of software, used a photo from the web for a blog post, or even tinkered with the code that runs your website? Chances are, you’ve bumped into something called a copyleft license, whether you knew it or not. It’s one of the most important ideas in the digital world, but it can seem a bit baffling at first.

You might think of copyright as a big, heavy lock. It’s designed to stop people from copying, sharing, or changing someone’s work without permission. It’s all about control. Well, copyleft is the clever key that unlocks it. But here’s the twist: it doesn’t just throw the door wide open for a free-for-all. Instead, it sets one big rule: if you use or change this work, you have to share your new version with the same key. You have to pass the freedom on.

It’s a bit like a community cookbook. Someone adds a recipe for a brilliant Victoria sponge. The rule of the cookbook is that anyone can use the recipe, tweak it—maybe add a bit of lemon curd—but if they do, they must put their new-and-improved recipe back into the book for everyone else to enjoy. You can’t take the recipe, improve it, and then start selling it under your own brand while keeping the secret ingredient to yourself.

Copyleft is the legal magic that makes this happen. It’s a powerful idea that turned the software world upside down and is now changing everything from art and music to science and engineering. This guide will walk you through what it is, where it came from, and why it matters to all of us here in the UK.

First, A Quick Refresher: What Is Copyright?

Before we dive into the deep end with copyleft, let’s make sure we’re all on the same page about copyright.

In the UK, copyright is an automatic right that protects original work. The moment you write a poem, snap a photo, compose a song, or write a line of code, copyright law automatically protects it. It’s your work. This means that, for a certain period (usually your lifetime plus 70 years), you’re the only one who gets to decide who can:

Copy your work.
Distribute it to others (sell it, give it away).
Adapt it or make new works based on it.
Perform or display it in public.

Think of it as the default setting. Copyright law essentially puts up a big “All Rights Reserved” sign. It’s designed to help creators make a living from their work, which is a brilliant and necessary thing. But in the 1980s, some computer programmers started to feel that this default setting was getting in the way of something equally important: collaboration and progress.

In the early days of computing, programmers often shared their code freely. It was a small, academic world, and helping each other out was just how things were done. But as software became a big business, companies started locking their code away. It became a secret—what’s known as proprietary software. You could buy the software, but you couldn’t see how it was made, you couldn’t fix it if it went wrong, and you certainly couldn’t adapt it to do something new.

For one programmer in particular, this just wouldn’t do.

Enter Copyleft: Flipping Copyright on Its Head

The hero of our story is a fiercely intelligent and uncompromising programmer from America named Richard Stallman. In the early 1980s, Stallman was working at the Massachusetts Institute of Technology (MIT). He got frustrated with a new printer whose software was proprietary. He had the skill to fix a bug in it, but the company refused to give him the source code—the human-readable instructions that tell the computer what to do.

For Stallman, this was a moral issue. He believed that software should be free—not free as in “free beer,” but free as in “freedom” (or libre, as in free speech). He felt everyone should have the freedom to see, change, and share the software they used.

So, he came up with a brilliantly clever idea. Instead of fighting against copyright law, he would use it to achieve the opposite goal. This idea was copyleft.

Here’s the simple version:

You start with a copyrighted work (like a piece of software).
You give everyone permission to run it, copy it, change it, and share it.
You add one crucial condition: If someone creates a new work based on yours (a “derivative work”), they must release their new work under the exact same copyleft license.

This is the “share and share alike” rule. It uses the power of copyright law to force people to keep the work free and open for everyone, forever. The freedom becomes infectious. That’s why some people call copyleft licenses viral licenses, though proponents prefer the term reciprocal. It’s not about infecting other software; it’s about sharing with the community that helped you.

The Story of Copyleft: A Rebel with a Cause

Richard Stallman didn’t just have an idea; he started a movement. In 1983, he announced the GNU Project. The goal was ambitious, to say the least: to create an entire computer operating system that was completely free software. “GNU” stands for “GNU’s Not Unix,” a recursive acronym typical of hacker humour at the time. Unix was the dominant proprietary operating system, and Stallman wanted to build a free alternative.

To give his project a legal backbone, Stallman and his team at the newly formed Free Software Foundation (FSF) wrote a license to enshrine the principles of copyleft. In 1989, they released the first version of the GNU General Public License (GPL).

The GPL was revolutionary. It was the first license to legally codify the idea of copyleft. It laid out what it called the Four Essential Freedoms:

Freedom 0: The freedom to run the program for any purpose.
Freedom 1: The freedom to study how the program works and change it to make it do what you wish. (Access to the source code is a precondition for this).
Freedom 2: The freedom to redistribute copies so you can help your neighbour.
Freedom 3: The freedom to distribute copies of your modified versions to others. By doing this, you can give the whole community a chance to benefit from your changes. (Access to the source code is a precondition for this).

These four freedoms are the heart of the free software movement and the legal foundation of copyleft. The GPL ensures that any software released under it, and any software built upon it, will always grant these freedoms to its users.

How Does Copyleft Actually Work? The Nitty-Gritty

So, how does this all work in practice? Let’s get into the details without getting lost in legal jargon.

Imagine you’re a developer and you’ve written a fantastic piece of software—let’s call it ChitChat, a new messaging app. You want everyone to be able to use and improve it, but you don’t want a big corporation like MegaCorp to take your code, make a few tweaks, and sell it as their own closed-source MegaChat.

This is where you’d use a copyleft license like the GNU GPL.

You declare your code is under the GPL. You include a copy of the license text with your source code. This is your legal declaration.
Anna downloads ChitChat. She’s free to use it, look at the code, and even change it. The GPL guarantees her this.
Anna is a brilliant coder. She adds a new feature that lets users translate messages in real-time. She’s created a “derivative work.”
Anna wants to share her new version. The GPL says she absolutely can. But because she used your GPL-licensed code, her new version must also be licensed under the GPL. She has to share her source code for the translation feature and grant everyone the same Four Freedoms you did.
Now, MegaCorp comes along. They see Anna’s amazing new version. They could use it to build their own MegaChat. But if they do, the GPL’s copyleft provision kicks in. They would be legally required to release the entire source code for MegaChat under the GPL.

For most big companies that make their money from proprietary software, this is a non-starter. Their whole business model is based on keeping their code secret. So, the GPL acts as a kind of protective shield, keeping the software and all its future versions within the world of free, open-source software.

It’s a legal hack of beautiful simplicity. It doesn’t create new laws; it just uses existing copyright law in a way its creators probably never imagined.

Types of Copyleft: Not a One-Size-Fits-All Jumper

Just as there are different styles of jumpers for different weather, there are different “strengths” of copyleft for different situations. The two main types are strong and weak copyleft.

Strong Copyleft

This is the purest form of copyleft, embodied by the GNU GPL.

A strong copyleft license says that any work derived from the original must be released under the same license. This applies even if your code only makes up a tiny part of the new, larger work. The “viral” or “reciprocal” nature is at its most powerful here. It ensures that the entire software ecosystem built around the original code remains free.

Simplified Explanation: Think of it like a pot of red paint. If you have a project made of hundreds of LEGO bricks, and just one of them is a strong-copyleft red brick, the whole model is now considered “red.” You have to share the building instructions for the entire thing.
Detailed Explanation: Legally, the key is the definition of a “derivative work.” With the GPL, if you link your code with GPL-licensed code to create a single, combined program, the entire program is generally considered a derivative work and must be licensed under the GPL.
Famous Examples: The Linux kernel, WordPress, the GIMP image editor, and the VLC Media Player.

Weak Copyleft

Sometimes, strong copyleft is too restrictive. What if you want to create a free software library—a reusable chunk of code—that proprietary software developers can also use? If you used the GPL, any application that used your library would have to become GPL-licensed itself, which would scare away most commercial developers.

This is where weak copyleft comes in.

A weak copyleft license has the same “share and share alike” requirement for any direct modifications to the code itself. However, it allows other, separate pieces of software to link to it without having to adopt the copyleft license.

Simplified Explanation: Let’s go back to our LEGO model. A weak-copyleft brick is a special blue brick. If you change the blue brick itself—say, by painting a smiley face on it—you have to share your modified blue brick under the same rules. But you can happily plug that blue brick into your otherwise proprietary LEGO spaceship, and you don’t have to share the plans for the whole spaceship. You only have to share the changes to the blue brick.
Detailed Explanation: These licenses draw a distinction between modifying the library and merely using it. The copyleft provision applies to the former but not the latter. This allows proprietary software to benefit from and link against open-source libraries without being “infected” by the copyleft terms.
Famous Examples: The GNU Lesser General Public License (LGPL) is the most famous example, designed specifically for software libraries. The Mozilla Public License (MPL), used for Firefox, is another popular weak copyleft license.

Copyleft vs. Permissive Licenses: Two Flavours of Open Source

It’s important to know that copyleft isn’t the only game in town when it comes to open-source software. The other major category is permissive licenses.

Permissive licenses, like the MIT License and the Apache License, are extremely simple. They basically say: “Here’s my code. Do whatever you want with it. You can copy it, change it, sell it, make it proprietary—I don’t mind. Just don’t sue me if it breaks, and maybe give me a little credit.”

There is no “share and share alike” requirement. This is why you’ll find MIT-licensed code inside products from Apple, Google, and Microsoft. They can use the code without having to share their own secret sauce.

The choice between copyleft and permissive often comes down to philosophy:

Copyleft licenses prioritise the freedom of the software and the community. They ensure that the work, and all derivatives of it, remain free forever. The goal is to build a lasting commons of shared code.
Permissive licenses prioritise the freedom of the developer. They give the person using the code maximum flexibility to do whatever they want, including making it proprietary.

Neither is inherently “better”—they just have different goals. Copyleft is about freedom with responsibility, while permissive is about freedom with few strings attached.

Why Would Anyone Choose Copyleft? The Pros and Cons

Choosing a license is a big decision. Here’s a look at why someone might—or might not—choose copyleft.

The Benefits of Copyleft

Builds a Community: By forcing everyone to contribute their improvements back, copyleft fosters a vibrant, collaborative ecosystem. Everyone benefits from everyone else’s work. The Linux operating system, with thousands of contributors worldwide, is the ultimate testament to this.
Prevents Corporate Hijacking: It stops a company from taking a community-built project, making it proprietary, and cutting the original creators out. This is often called “Embrace, Extend, Extinguish,” a tactic that copyleft is specifically designed to prevent.
Guarantees Enduring Freedom: It ensures that the software will always remain free for all users. The freedoms can’t be taken away in a future version.
Levels the Playing Field: A small start-up can use powerful, copylefted software without fear that a huge competitor can just take their improvements and out-muscle them. Everyone has to play by the same sharing rules.

The Drawbacks of Copyleft

Commercialisation Can Be Tricky: Companies that want to sell software in the traditional, proprietary way often can’t use strong copyleft code in their products. This can limit adoption in some corporate environments. (Though many companies, like Red Hat in the UK, have built hugely successful business models around supporting and servicing copylefted software).
License Incompatibility: Sometimes, the rules of one copyleft license can clash with the rules of another, making it legally impossible to combine code from two different projects. This is sometimes jokingly called “license hell.”
Ideological Baggage: For some, the free software movement’s philosophy is too rigid. They just want to write code and share it without worrying about the politics of software freedom.

Copyleft in the Wild: Famous Examples You Use Every Day

You are almost certainly using copylefted software right now. It runs the modern world.

The Linux Kernel (GPLv2): This is the absolute giant of copyleft. It’s the core of the Linux operating system, which runs the vast majority of the world’s web servers, supercomputers, and stock exchanges. It’s also the foundation of Android, the operating system on billions of smartphones.
WordPress (GPLv2): This content management system powers over 40% of all websites on the internet. Its copyleft nature means there is a massive community of developers creating and sharing themes and plugins.
VLC Media Player (GPLv2): If you’ve ever needed to play a video file in a weird format that nothing else would touch, you’ve probably used VLC. It’s a shining example of a community-driven project that’s better than almost any commercial alternative.
Audacity (GPLv2): A hugely popular free audio editor and recorder, used by podcasters, musicians, and journalists all over the UK and the world.

Copyleft Beyond Software: A Spreading Idea

The brilliant idea of using a field’s own rules to enforce sharing was too good to stay confined to software. The concept has been adapted for all kinds of creative and scientific works.

The most famous example is Creative Commons (CC). Founded in 2001, Creative Commons provides a set of simple, free licenses that creators can use for their work. One of the most popular is the Share-Alike (SA) license.

The Creative Commons Attribution-ShareAlike (CC BY-SA) license is the copyleft of the creative world. It lets people copy, share, and adapt a work, even for commercial purposes, as long as they give credit to the original author and—you guessed it—release their new version under the same CC BY-SA license.

Wikipedia is perhaps the most famous user of this license. All of its text is licensed under CC BY-SA, which means the world’s largest encyclopedia will always remain a free and open resource that anyone can build upon, as long as they share back.

The copyleft principle is also being applied to:

Open Hardware: Designs for physical objects, from circuit boards to 3D-printable machines, are being released under copyleft-style licenses.
Science and Research: Scientists are using similar agreements to ensure that research data and findings remain open and accessible to all.
Art and Music: Musicians and artists use Share-Alike licenses to encourage remixing and collaboration while ensuring their work remains part of the creative commons.

Common Myths and Misunderstandings

Because it’s a counter-intuitive idea, a few myths have sprung up around copyleft. Let’s bust them.

Myth 1: Copyleft means “no copyright.” Busted: This is the biggest misconception. Copyleft depends on copyright to work. It is a creative use of copyright law, not an abandonment of it. Without copyright, there would be no legal mechanism to enforce the “share and share alike” rule.

Myth 2: You can’t make money with copyleft software. Busted: This is completely wrong. You are perfectly free to sell copylefted software. You can charge for your time, for support, for convenience (like providing an easy installer), or for hosting. The UK company Red Hat became a multi-billion-dollar business primarily by selling support and services for the copylefted Linux operating system. The only thing you can’t do is sell it as a proprietary, closed-source product. You must always provide the source code and the associated freedoms.

Myth 3: Copyleft is anti-business. Busted: It’s more accurate to say that copyleft is anti-proprietary business models. It challenges the traditional way of selling software, but it enables other business models based on service, support, and customisation. For many modern tech companies, using and contributing to open-source software, including copylefted projects, is a core part of their strategy.

The Future of Copyleft: Still Relevant in a Cloudy World?

Copyleft was born in an era when software was something you installed on your own computer. But today, much of our computing happens “in the cloud,” on services like Google Docs or Microsoft 365. You don’t run the software; you just access it through your web browser.

This created a loophole. The original GPL’s copyleft provision was triggered when you distributed the software. But if a company like Google heavily modifies a GPL program to run its own internal services, it isn’t “distributing” it to users. It’s just letting them interact with it over a network. This means they don’t have to share their changes.

To close this “cloud loophole,” the Free Software Foundation created the Affero General Public License (AGPL). It adds one extra clause: if you run a modified version of the software on a server and let users interact with it over a network, you must also offer them the source code.

The AGPL remains controversial, but it shows that the principles of copyleft are still evolving to meet the challenges of the modern digital world. The debate continues about how to best ensure user freedom in an age of artificial intelligence, massive cloud platforms, and the Internet of Things.

Conclusion: The Enduring Power of Sharing

Copyleft started as a clever legal hack, born out of one programmer’s frustration. But it grew into something much bigger: a philosophy, a movement, and the legal engine behind some of the most important technology we use every single day.

It’s a powerful reminder that “free” doesn’t have to mean a free-for-all. By using the rules of ownership to build a community of sharing, copyleft creates a digital commons—a shared resource of knowledge and tools that is constantly being improved by everyone who uses it.

From the servers that deliver this webpage to you, to the phone in your pocket, to the encyclopedia you use for your homework, copyleft is quietly working in the background. It’s the legal backbone of a more open, collaborative, and free digital world. And it all started with a simple, powerful idea: if you take something from the community pot, you should put something back.

What is a Copyleft License? The Ultimate UK Guide to Sharing Your Work

First, A Quick Refresher: What Is Copyright?

Enter Copyleft: Flipping Copyright on Its Head

The Story of Copyleft: A Rebel with a Cause

How Does Copyleft Actually Work? The Nitty-Gritty