Examples of dual-licensing practises
The holder of the copyright of a program can release it under the terms of more than one licence. So-called Dual-Licencing are popular in the commercial field, where companies want to involve the Community but want to retain the copyright on the whole software. In order to be able to re-licence the software, a company must own the copyright of every line of the source code. This article tries to show some examples on how this is handled in practice.
It is possible to give the Copyright to other, without having to resign the own Copyright. OpenOffice.org relies on this property of the copyright. In fact, the copyright of a modification is owned by both the contributor and by Sun Microsystems. Every contributor has to sign a "Joint Copyright Assignment" in which he states that he owns all the copyright of the contribution and he shares the Copyright of the contributed code with Sun ("shared copyright").
MySQL AB has a slightly different approach. Almost every part of the software is written internally but MySQL AB, but external contributions are also welcome. The company has started a MySQL Code Contribution Program. which co-ordinates contributions. The MySQL Contributor License Agreement states that the author of submitted code must cede all right on his code (and grant a licence on patents pending on the submitted code), and gets a "broad licence" on his own code back.
The Mozilla Public License doesn't provide any transfer of copyright, as I understood it. The MPL allows you to mix proprietary code with Open Source Code, as long as the copyright of the code which was released under the terms of the MPL isn't changed. This ensures that code remains Open, once it is released under the MPL, but it doesn't require to disclose linked-in proprietary code (cf. "viral effect of the GPL").
The licences reflect to what degree a company wants to give away the control of a software project. MySQL AB apparently doesn't need necessarily many external contributions and thus the concessions to contributors is relatively low. But the company secures the maximum of rights of contributions. Other examples, such as OpenOffice.org and Mozilla can barely survive without an active community, and this can be seen in the way the contributor is not required to give away his copyright on his code. Netscape (whoever it is now) is able to mix its code with the Mozilla code base and to distribute binaries, but is not alone with this right: everyone can do the same. Sun Microsystems on the other hand shares the copyright with the contributor, but is in the singular situation of holding the copyright of the whole software, and has thus the exclusive right to re-licence the code.