The Blue Oak Guide to Copyleft
This short guide gives a brief introduction to copyleft licenses, an important kind of open software license. Understanding copyleft licenses at a high level unlocks a world of new software, from Linux to WordPress to LibreOffice and more.
Permissive and Copyleft
Experts distinguish two kinds of open software licenses:
Permissive licenses give everyone the right to do nearly anything with software, for free. That includes building new software that made available under commercial terms, rather than open terms, or kept within an organization.
The council maintains a list of permissive licenses, to help you identify them in the wild.
Copyleft licenses work like permissive licenses with a catch: they require sharing and licensing the source code for new software built with copyleft-licensed software as open software, too. For this reason, copyleft licenses are sometimes called “share-alike” licenses, though “copyleft” is the term you’ll hear most in software.
Depending on how you use software, copyleft licenses might require sharing and licensing other software alike. So copyleft licenses pose some questions that permissive licenses don’t:
When does the license require you to share?
What source code does the license require you to share?
How do you have to license the source code you have to share?
How do you have to make that source code available to others?
In theory, copyleft licenses can answer each of these questions differently. But in practice, copyleft licenses tend to fall into a few loose families that give similar answers.
If you have a specific question about a particular copyleft license, you should seek a specific answer in the terms of the license. But slightly oversimplifying in this way, to start, helps both in learning and in writing policies, like our license policies for small and larger organizations.
These family lists are also available as JSON.
Skip To: Weak Strong Network Maximal
Weak Copyleft Family
Weak copyleft licenses require sharing changes and additions to the licensed software when you give copies to others.
Common Public License 1.0 (
Eclipse Public License 2.0 (
Erlang Public License v1.1 (
IBM Public License v1.0 (
GNU Library General Public License v2 only (
GNU Library General Public License v2 or later (
GNU Lesser General Public License v2.1 only (
GNU Lesser General Public License v2.1 or later (
GNU Lesser General Public License v3.0 only (
GNU Lesser General Public License v3.0 or later (
Mozilla Public License 1.0 (
Mozilla Public License 1.1 (
Mozilla Public License 2.0 (
Microsoft Reciprocal License (
Sun Public License v1.0 (
Strong Copyleft Family
In addition to the requirements of weak copyleft licenses, strong copyleft licenses require you to share larger programs that you build with the licensed software when you give copies to others.
BSD Protection License (
Creative Commons Attribution Share Alike 1.0 Generic (
Creative Commons Attribution Share Alike 2.0 Generic (
Creative Commons Attribution Share Alike 2.5 Generic (
Creative Commons Attribution Share Alike 3.0 Unported (
Creative Commons Attribution Share Alike 4.0 International (
GNU General Public License v2.0 only (
GNU General Public License v2.0 or later (
GNU General Public License v3.0 only (
GNU General Public License v3.0 or later (
Sleepycat License (
Network Copyleft Family
In addition to the requirements of strong copyleft licenses, network copyleft licenses require you to share larger programs that you build with the licensed software not just when you give copies to others, but also when you run the software for others to use over the Internet or another network.
Affero General Public License v1.0 only (
Affero General Public License v1.0 or later (
GNU Affero General Public License v3.0 only (
GNU Affero General Public License v3.0 or later (
Common Public Attribution License 1.0 (
Non-Profit Open Software License 3.0 (
Open Software License 1.0 (
Open Software License 1.1 (
Open Software License 2.0 (
Open Software License 2.1 (
Open Software License 3.0 (
Maximal Copyleft Family
Maximal copyleft licenses answer the question “When does the license require you to share?” differently than other families. Maximal copyleft licenses require you to share software you make with others, and to license that software alike when you do.
Maximal copyleft licenses may require sharing just changes and additions to the licensed software, like weak copyleft licenses, whole programs, like strong copyleft licenses and network copyleft licenses, or both.