What is ShareAlike compatibility and why is it important?
The ShareAlike licenses are designed to ensure that the freedoms associated with a licensed work survive as the work is adapted by others and that those freedoms attach to adaptations of the work as well. There are other copyleft licenses designed for this same purpose, but unfortunately, the works licensed under these other licenses are not compatible with works licensed under BY-SA. Creative Commons created the ShareAlike compatibility process as a means to solving this problem. The process allows CC to evaluate other copyleft licenses to determine if they are similar enough to BY-SA to meet licensor expectations to be deemed compatible. BY-SA Compatible Licenses are listed here.
How does ShareAlike compatibility work?
Once a license is declared compatible with BY-SA, anyone adapting BY-SA works can apply that license to their own contributions to the adaptation. With version 4.0, future versions of BY-SA are automatically compatible. Non-CC licenses have to undergo the ShareAlike compatibility process to be officially designated as a BY-SA Compatible License by CC. In either case, if someone applies a compatible license to an adaptation, both the original BY-SA license and the compatible license apply to the adaptation and must be followed. However, downstream users of the adaptation may choose to look ‘only at the compatible license (that is, the last license applied) to determine what their attribution and ShareAlike obligations are under both licenses when they reuse the work.
What does one-way or two-way compatibility mean?
One-way compatibility means that you may adapt work under one license (X) and apply a second license (Y) to your own contributions, but you may not adapt work under the Y license and apply the X license to your contributions. For example, CC BY is one-way compatible with BY-SA. You may adapt a BY work and apply BY-SA to your contributions, but you may not adapt a BY-SA work and apply BY to your contributions.
Two-way compatibility means that you may adapt work under one license (X) and apply a second license (Y), and vice versa.
For ShareAlike compatibility, a license listed as a ShareAlike Compatible License on the compatible licenses page is two-way compatible unless otherwise noted.
What licenses are compatible with BY-SA?
Different versions of BY-SA have different compatibility mechanisms. Each version is addressed below.
Why did CC choose the Free Art License to be the first BY-SA Compatible License?
Drafted in 2000, the Free Art License was one of the original copyleft licenses designed for content, not code. The latest version (1.3) was published in 2007 with an eye toward compatibility with CC licenses. The FAL 1.3 is very similar to BY-SA 4.0 in spirit and substance, and raised no significant obstacles to compatibility. For a detailed explanation of the policy decisions involved in the compatibility determination, see here.
Why did CC decide to pursue compatibility with a software license (GPLv3)?
In some niche areas such as gaming, content is increasingly blended with code in ways that make it difficult for downstream users to differentiate between the two. Developers in these fields sometimes abstain from integrating BY-SA content into GPL-licensed software projects because of potential conflict between the two copyleft licenses, or are adapting and incorporating BY-SA content without accounting for the incompatibility between the licenses. This obstacle to reuse and remix of BY-SA content in projects published under a license so similar in spirit and substance was at the heart of the problem the compatibility mechanism in BY-SA was designed to solve.
Together with the Free Software Foundation, CC has pursued a limited form of compatibility to address the problem. Because the GPLv3 is designed for software and CC discourages use of its licenses for software, there are special considerations for those seeking to take advantage of this compatibility mechanism. Please read those considerations carefully before deciding to take advantage of the one-way compatibility mechanism.