Dual license

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Licenses are granted by copyright holders to grant exceptions of copyright law to users for a work. Works can be made available under a single license, but the copyright holder has the option of making their work available under any number of licenses.

[edit] Use in free software

Dual licensing is used by the copyright holders of some free software packages advertising their willingness to distribute using both a copyleft free-software license and a non-free software license. The latter license typically offers users the software as proprietary software or offers third-parties the source code without copyleft provisions. Copyright holders are exercising the monopoly they're provided under copyright in this scenario, but also use dual licensing to discriminate the rights and freedoms different recipients receive.

Such licensing allows the holder to offer customisations, early releases, generate other derivative works or grant rights to third parties to redistribute proprietary versions all while offering everyone a free version of the software. Sharing the package as copyleft free software can benefit the copyright holder by receiving contributions from users and hackers of the free software community. These contributions can be the support of a dedicated user community, word of mouth marketing or modifications that are made available as stipulated by a copyleft license. However, a copyright holder's commitment to elude copyleft provisions and advertise proprietary redistributions risks losing confidence and support from free software users.[1][2]

A second use of dual-licensing with free software is for license compatibility, allowing code from differently licensed free software projects to be combined or to provide users the preference to pick a license.

Examples of dual licensed free software are:

Asterisk PBX
Berkeley DB by Sleepycat Software
CodeSynthesis XSD by Code Synthesis
DB4O
EiffelStudio by Eiffel Software
eXo Platform
Internet Communications Engine
Mono
Mozilla Application Suite by the Mozilla Foundation
MySQL by MySQL AB
Perl
Qt by Trolltech
Sendmail

[edit] In proprietary works

Dual licensing can also be used to segregate a market. By splitting people into categories such as home users, professional users, and academic users, copyright holders can set different prices for each group.