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Dual licensing is a business model for free and open source software, based on releasing it under two licenses. One is a proprietary software license, which allows the possibility of creating proprietary applications derived from it, while the other license is a copyleft free software/open source licence, thus requiring any derived work to be released under the same licence. The copyright holder of the software then usually gives away the free/open source version of the software at no cost, and charges licensing fees for the proprietary version.

Since in most cases only the copyright holder can change the licensing terms of a software, dual licensing is mostly used by companies that control software exclusively.

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.

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]

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
• sipX by Pingtel

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

• Dual Licensing Advisory from OSS Watch
• Article "Does dual licensing threaten free software?" by Glyn Moody
• 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 Schemes
• How dual licensing works
• GPL violations

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