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’s made available under different, commercial license terms, rather than open terms, or that’s kept secret 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.
Copyleft Questions
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:
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When does the license require you to share?
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What source code does the license require you to share?
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How do you have to license the source code you have to share?
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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.
Copyleft Families
Version 9
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.
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Common Development and Distribution License:
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Common Public License:
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Eclipse Public License:
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Erlang Public License:
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IBM Public License:
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GNU Library General Public License:
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Mozilla Public License:
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Microsoft Reciprocal License:
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Sun Public License v1.0:
Strong Copyleft Family
Strong copyleft licenses require you to share both the licensed software (like the weak copyleft licenses), and larger programs that you build with the licensed software, when you give copies to others.
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BSD Protection License:
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copyleft-next:
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GNU General Public License:
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Q Public License:
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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.
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Affero General Public License:
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Apple Public Source License:
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Common Public Attribution License:
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European Union Public License:
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Non-Profit Open Software License:
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Open Software License:
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RealNetworks Public Source License:
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Server Side Public License:
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.
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Cryptographic Autonomy License:
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The Parity Public License:
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Reciprocal Public License:
Non-Software Copylefts
There are a variety of copyleft licenses that apply to things other than software, like documentation or databases. (These are also often known as “Share Alike” licenses.) This guide does not cover those in depth, and Blue Oak strongly recommends against using these for software unless necessary to match an existing community’s policies. For more information, you might refer to the Open Knowledge Foundation’s list of Conformant Licenses, particularly those labeled “SA” (Share Alike).