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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.

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 proproprietary redistributions risks losing confidence and support from free software users.^**

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
• eXo Platform
• Internet Communications Engine
• Mono
• Mozilla Application Suite by the Mozilla Foundation
• MySQL by MySQL AB
• Perl
• Qt by Trolltech
• Sendmail

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.

External links

• Dual Licensing Advisory from OSS Watch
• Article "The Dual-Licensing Model" by Don Marti
• Article "Dual Licensing: Having Your Cake and Eating It Too" by Philip H. Albert
• Article "Dual-Licensing Open Source Business Models" by Heather Meeker
• Article "How to use open source as a power marketing tool" by John Koenig
• Paper "Dual Licensing in Open Source Software Industry" by Mikko Välimäki
• Dual Licensing Strategies from Open Source Law Blog
• Dual Licensing Schemes
• How dual licensing works
• SOE launches dual licensing program
• GPL violations

Software licenses

Duales Lizenzsystem | Licencia dual