Dual-license

Multi-licensing is the practice of distributing software under two or more different sets of terms and conditions. This may mean multiple different software licenses or sets of licenses. Prefixes may be used to indicate the number of licenses used, e.g. dual-licensed for software licensed under two different licenses.

When software is multi-licensed, recipients can typically choose the terms under which they want to use or distribute the software, but note, that the simple presence of multiple licenses in a software package or library does not necessarily indicate that the recipient can freely choose one or the other. In some cases, especially when the software has multiple origins, all the accompanied licenses apply at the same time. The applicability of the different licenses has to be individually checked. The distributor may or may not apply a fee to either option. The two usual motivations for multi-licensing are license compatibility and market segregation based business models.

Multi-licensing is commonly done to support free software business models in a commercial environment. In this scenario, one option 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 license, thus requiring any derived work to be released under the same license. The copyright holder of the software then typically provides the free version of the software at little or no cost, and profits by selling proprietary licenses to commercial operations looking to incorporate the software into their own business. This model can be compared to shareware.

Since in most cases, only the copyright holder can change the licensing terms of a software, multi licensing is mostly used by companies that wholly own the software which they are licensing. Confusion may arise when a person outside the company creates additional source code, using the less restrictive license. Because the company with the official code is not the copyright holder of the additional code, they may not legally include this new work in their more restrictively licensed version. Companies may demand outside developers agree to a contributor license agreement, before accepting their work in the official codebase and source code repositories.

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