Permissive free software licence
A permissive free software licence is a free software license that applies to an otherwise copyrighted work. It also offers many of the same rights found in free software licenses when releasing a work to the public. In contrast to non-permissive copyleft (reciprocal) licenses such as the GNU General Public License, any copies and derivatives of the source code created under permissive licenses may be made available on terms that are more restrictive than those of the original license. For example, source code released under a permissive software license does not have the requirement that all distributed derivatives also be made available to the public.
Well-known examples of permissive licenses include the MIT License and the BSD licenses.
 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 licenses are not actually equivalent to releasing a work into the public domain.
Permissive licenses often do stipulate some limited requirements, such as that the original authors must be credited (attribution). If a work is truly in the public domain, this is usually not legally required, but a United States copyright registration requires disclosing material that has been previously published, and attribution may still be considered an ethical requirement in academia.
 GPL compatibility
Some permissive free software licenses contain clauses that require advertising materials to credit the copyright holder. Licenses with an advertising clause include the 4-clause BSD license, the PHP License, and the OpenSSL License. These licenses, although they are permissive free software licenses, are incompatible with the widely used GNU General Public License.
Examples of permissive free software licenses 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.
Some licenses do not allow derived works to add a restriction that says a redistributor cannot add more restrictions. The purpose of such clauses is to disallow redistribution using the GPL or similar copyleft licenses. There are many examples such as the CDDL and MPL. However such restrictions also make the license incompatible with the BSD license.
 See also
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