Blue Oak Council

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:

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 question about a specific copyleft license, you should seek a specific answer in the terms of that 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

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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.

Strong Copyleft Family

In addition to the requirements of the 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.

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.

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.

Non-Software Copyleft

There are also copyleft licenses that apply to things other than software, like documentation or databases. These are also known as “share-alike” licenses.

This guide does not cover those licenses, and Blue Oak strongly recommends against using them for software unless necessary to match an existing community’s policies. For more information, you refer to the Open Knowledge Foundation’s list of Conformant Licenses, particularly those labeled “SA” for “Share Alike”.