To enable collaboration between contributors, and to keep their contributions free for everyone to use, Wikivoyage content is released under a copyleft license called the Creative Commons Attribution-ShareAlike 3.0 License (sometimes called the by-sa license). Another, similar license, the GNU Free Documentation License (or GFDL), is used by other Free Content wikis to keep their content Free. Wikivoyage doesn't use the GFDL; see Why Wikivoyage isn't GFDL for details.
Some contributors may wish to release their contributions under another license (as well as the by-sa) such as the GFDL, in order to make their work accessible for these other sites (or for other reasons). This is called dual licensing. This page describes how to dual-license your work for Wikivoyage, as well as what the pitfalls of dual licensing can be. It's provided here as a service for those Wikivoyagers who insist on dual-licensing their work, and a clarification for other Wikivoyagers who have to edit and modify that work.
NOTE: The contributors to this article are either a) not lawyers or b) not giving you legal advice. If you need legal advice on this issue, please contact a lawyer familiar with Free Content licensing issues.
Advantages of dual licensing 
- You may have other personal reasons to be attached to the GFDL or other licenses.
- Dual licensing allows material to be shared between projects, that use different incompatible licenses. The creater of the original version can submit his work under different licenses to different projects. But for updates to flow between the different projects, dual licensing is necessary.
Limitations of dual licensing 
The major problem with dual licensing is that it's confusing for readers, re-distributors, and other contributors. It makes them go read pages like this one to figure out what their rights and responsibilities are. Because it's confusing, it keeps us from working together on articles -- which is one of the main reasons we use a copyleft license in the first place!
Mechanics of dual licensing 
The following describes the mechanics of dual licensing. It refers to the GFDL specifically, but much of the discussion would apply to any other license.
- It is possible for the original creator of a Work (that is, a Wikivoyage article, image, or other media object) to release that work under the by-sa license as well as another license such as the GFDL.
- A contributor making a Derived Work (editing or adding to the article, resizing or fixing up an image, etc.) must choose one of the licenses or both -- either the GFDL, the by-sa, or both. Because of limitations in our software, if that contributor chooses to use only the GFDL to create a derived work, it can't be uploaded to Wikivoyage. (We don't have a way to separate out content that's not available under the by-sa license. If we need it, we might add that "feature", but we don't have it yet.) So, if a contributor edits and saves an article in Wikivoyage, it is explicitly licensed under by-sa only, unless otherwise specified.
- Once a Work has been edited by another contributor who doesn't want to dual-license it, it is under the by-sa license only. Nobody -- not even the original Creator -- can create a derived work from it that is dual-licensed.
The upshot: work can be dual-licensed in Wikivoyage only if one person has worked on the article or image, and if all subsequent contributors have also chosen to dual-license it.
How to dual-license your original contributions to Wikivoyage 
First, remember that dual-licensing is your choice as a contributor. You cannot expect other collaborators to keep an article or image dual-licensed.
If you have an article in Wikivoyage that is your original work, and that no one else has worked on, you can dual license it. Simply add the following text to the talk page for the article or images:
- All content on Wikivoyage is available under the terms of the Creative Commons Attribution-ShareAlike 3.0 license. The author of this content has also granted you permission to use the content under a different license (see [[Project:dual licensing|]] for details), if you so choose.
- Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. See the [[Project:full text of the GNU Free Documentation License, Version 1.2|]] for details.
Once another contributor edits the article without explicitly dual-licensing the update, its latest version is (as mentioned above) no longer available under the GFDL. If you wish, you can make a link on the Talk: page pointing to the last GFDL'd version. This is your responsibility.
How to dual-license existing guides 
You can't dual-license a guide that's already been edited by other people who didn't dual-license it. But if it's already been dual-licensed under the GFDL and by-sa 3.0, you can dual-license it under those two licenses. (It's not clear that you can dual-license it under other pairs of licenses, though.)
If you agree to keep a work dual-licensed, add a note on the talk page to that effect. If you don't, other editors can and should assume that you'd don't agree to the dual license, and will remove the dual-licensing statement.
Editing a dual-licensed article 
If you edit an article or image that's dual-licensed, then please change the above license statement on the Talk: page to specify that only the older version(s) are dual-licensed. You can replace the license statement as so:
- All content on Wikivoyage is available under the terms of the Creative Commons Attribution-ShareAlike 3.0 license. Older versions of this document were also available under the [[Project:full text of the GNU Free Documentation License, Version 1.2|GNU Free Documentation License, Version 1.2]] (GFDL). The most recent version available under the GFDL is [http://www.wikivoyage.org/wiki/wiki.phtml?title=Title&oldid=OLDID here]. See [[Project:dual licensing|]] for details.
This is a courtesy to the original author, who wished to distribute his/her work under a dual license. Alternately, you can simply delete the dual-licensing statement.
Dual licensing of Images and other media 
The rules are the same for images and text. However images are usually not edited by multiple authors and the current Wikivoyage software does not handle updating of images well.
If you want to dual-license images or other media so that they can be used with other projects, consider uploading a copy to commons.wikimedia.org and put the relevant licenses in the "Summary" field using mediaWiki codes. For example:
Make a link to the MediaWiki "original".