What is the difference between permissive and copyleft licenses?

Permissive and copyleft licenses are two categories of open-source licenses that differ in how they handle reuse and redistribution of code. Permissive licenses impose minimal restrictions, allowing developers to modify, distribute, and use code in proprietary projects as long as basic requirements like attribution are met. Copyleft licenses, on the other hand, require that any derivative work—software built using the licensed code—must also be distributed under the same copyleft terms, ensuring the code and its derivatives remain open source. The core distinction lies in how they enforce openness: permissive licenses prioritize flexibility, while copyleft licenses prioritize ensuring derivative works stay open.

Permissive licenses, such as the MIT License and Apache License 2.0, are designed to maximize adoption and reuse. For example, the MIT License requires only that the original copyright notice and license text accompany any redistribution, even in closed-source projects. Apache 2.0 adds explicit patent grants and protections against patent litigation. These licenses are popular in ecosystems like JavaScript (Node.js) or cloud-native tools (e.g., Kubernetes), where developers want their code widely used without legal friction. Companies often prefer permissive licenses for libraries or tools they want others to integrate into proprietary products, as it avoids forcing third parties to open-source their code.

Copyleft licenses, like the GNU General Public License (GPL), enforce stricter sharing requirements. The GPL mandates that any derivative work—such as a modified version of the code or software linking to it—must also be licensed under the GPL and include its source code. For instance, if you modify the Linux kernel (GPL-licensed) and distribute it, you must share your changes. Stronger variants like the AGPL extend this to software accessed over a network. Copyleft is common in projects like Git or WordPress, where maintaining open access to improvements is a priority. Developers choosing copyleft often aim to prevent proprietary forks while fostering community collaboration, though this can deter commercial adoption where closed-source components are required.