Permissive free software licences

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Permissive free software licences are free software licences for a copyrighted work that offer many of the same freedoms as releasing a work to the public domain. In contrast, copyleft licences like the GNU General Public License require copies and derivatives of the source code to be made available on terms no more restrictive than those of the original licence.

Well-known examples of permissive licences include the MIT License and the BSD licenses.

[edit] Comparison to public domain

Computer Associates Int'l v. Altai used the term "public domain" to refer to works that have become widely shared and distributed under permission, rather than work that was deliberately put into the public domain. However, such licences are not actually equivalent to releasing a work into the public domain, so such a term can be considered a misnomer.

[edit] GPL compatibility

Some permissive free software licences contain clauses that require advertising materials to credit to the copyright holder. Licences with an advertising clause include the 4-clause BSD license, the PHP License, and the OpenSSL Licence.

Licences with the advertising clause are incompatible with the GNU General Public License.

Examples of permissive free software licences without advertising clauses are the MIT License, the 3-clause BSD license, the Zlib License, and all versions of the Apache License except 1.0.