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License compatibility
A second use of multi-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 include the source code of Mozilla Application Suite and previously Mozilla Thunderbird and Mozilla Firefox, that have used tri-licensing under the Mozilla Public License (MPL) 1.1, GNU General Public License (GPL) 2.0 or GNU Lesser General Public License (LGPL) 2.1 before the latter upgraded to GPL-compatible MPL 2.0, making the tri-licensing unnecessary. Other examples are Perl, which is dual-licensed under the GPL or Artistic License, and Ruby, whose license contains explicit GPL dual licensing.
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