What are the most common open-source licenses?

The most common open-source licenses include the MIT License, Apache License 2.0, GNU General Public License (GPL), GNU Lesser General Public License (LGPL), and the BSD licenses. These licenses define how software can be used, modified, and distributed, with varying levels of restrictions and requirements. The MIT License is widely favored for its simplicity, allowing users to do almost anything with the code as long as the original license text is included. The Apache License 2.0 adds explicit patent grants and protections, making it popular for projects involving corporate contributors. The GPL family, including GPLv2 and GPLv3, enforces “copyleft,” requiring derivative works to remain open source under the same terms. The BSD licenses (2-clause and 3-clause) are permissive like MIT but include clauses about attribution and prohibiting the use of contributors’ names for endorsement without permission.

Permissive licenses like MIT, Apache, and BSD prioritize flexibility, enabling developers to use the code in proprietary projects or combine it with other licenses. For example, MIT-licensed code can be integrated into commercial software with minimal obligations, which is why projects like React (MIT) are widely adopted. Apache’s patent clause protects contributors and users from patent litigation, a key reason large organizations like Google use it for projects such as Kubernetes. In contrast, copyleft licenses like GPL and LGPL focus on ensuring software freedom. The GPL requires any modified or derivative work to be released under the same license, which is why Linux (GPLv2) remains open. The LGPL is a less restrictive variant, often used for libraries, allowing linking with proprietary code as long as the LGPL-licensed parts remain modifiable.

When choosing a license, developers should consider their goals. Permissive licenses encourage broad adoption and are ideal for projects seeking maximum reuse, including in proprietary software. Copyleft licenses are better for ensuring contributions stay open, though they can limit commercial use. License compatibility is also critical—for example, GPLv3 code can’t be mixed with Apache 2.0 code in the same project unless the latter is explicitly allowed. Projects depending on GPL libraries must also adopt GPL, which can complicate proprietary integrations. Tools like GitHub’s license chooser or the OSI’s recommendations help navigate these decisions. Ultimately, the license reflects the project’s values: MIT for simplicity, Apache for legal safeguards, or GPL for enforcing openness.