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 similar copyleft licenses to determine if they are similar enough to BY-SA to meet licensor expectations and hence be deemed compatible. BY-SA Compatible Licenses are listed here.
How does ShareAlike compatibility work?
Once a license is deemed 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. 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 if 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), but you may not adapt work under the Y license and apply the X license. For example, CC BY is one-way compatible with BY-SA. You may adapt a BY work and apply BY-SA, but you may not adapt a BY-SA work and apply BY.
Two-way compatibility means that you may adapt work under one license (X) and apply a second license (Y), or 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.
- For BY-SA 4.0, all future versions of BY-SA and all ports of BY-SA 4.0 or later are automatically compatible, as well as licenses explicitly deemed compatible pursuant to the ShareAlike compatibility process
- For BY-SA 3.0, all future versions of BY-SA and all ports of BY-SA 3.0 or later are automatically compatible, as well as licenses explicitly deemed compatible pursuant to the ShareAlike compatibility process
- For BY-SA 2.0, all future versions of BY-SA and all ports of BY-SA 2.0 or later are automatically compatible
- For BY-SA 1.0, only BY-SA 1.0 is compatible.
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 in spirit and substance, and raises 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 impossible for downstream users to differentiate the two. Developers in these fields were sometimes abstaining from integrating BY-SA content into GPL-licensed software projects because of potential conflict between the two copyleft licenses. This obstacle to reuse and remix of BY-SA content into 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 decided to pursue 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.
Example compatibility scenarios
- BY-SA 4.0 story is adapted into a screenplay and FAL 1.3 is applied
- both licenses apply, but reusers of the screenplay can attribute both authors and ShareAlike according to FAL terms
- FAL 1.3 image is adapted and BY-SA 4.0 is applied
- only BY-SA 4.0 applies to the adaptation; reusers of the BY-SA adaptation may look only at the terms of BY-SA 4.0 to determine their obligations
- BY-SA 3.0 song is synched with your original video and BY-SA 4.0 is applied
- both licenses apply, and reusers of the video must comply with the terms of both versions to the extent they differ [note: this is unlikely to have much practical significance; see here for details on how the license versions differ]
- BY-SA 4.0 assets are adapted into GPL game and GPLv3 is applied
- both licenses apply, but reusers of the GPL game can attribute and ShareAlike according to the GPL terms