A permissive free software licence is a class of free software licence with minimal requirements about how the software can be redistributed. This is in contrast to copyleft licences, which have reciprocity / share-alike requirements. Both sets of free software licences offer the same freedoms in terms of how the software can be used, studied, and privately modified. A major difference is that when the software is being redistributed (either modified or unmodified), permissive licences permit the redistributor to combine the licensed material with other licence terms, potentially adding further restrictions to a derived work, while copyleft licences do not allow further restrictions (among other possible differences). The term "permissive" as applied to software licensing is sometimes debatable in terms of specific terms and requirements, with occasional references to very weakly copyleft as "permissive". A more narrowly constrained term related to permissive licensing is copyfree[1], which implies distinct licence term requirements analogous to, but different from, those of free software.
Well-known examples of permissive free software licences include the MIT License, the BSD licences and the GNU Lesser General Public License. A well known copyleft licence is the GNU General Public License.
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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.
Permissive licences 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,[2] and attribution may still be considered an ethical requirement in academia.
Copyleft is "a general method for making a program or other work free, and requiring all modified and extended versions of the program to be free as well."[3] By comparison with permissive licences, copyleft licensing places more requirement in terms of distribution and combination with software under other licences.
Copycenter is a term originally used to explain the modified BSD licence, a permissive free software licence. The term was presented by Kirk McKusick, a computer scientist famous for his work on BSD, during one of his speeches at BSDCon 1999. It is a word play on copyright, copyleft and copy center.
| “ | The way it was characterized politically, you had copyright, which is what the big companies use to lock everything up; you had copyleft, which is free software's way of making sure they can't lock it up; and then Berkeley had what we called ‘copycenter’, which is ‘take it down to the copy center and make as many copies as you want.’ | ” |
The liberty to 'make as many copies as you want' is in fact also provided by all copyleft licences. However, unlike both copyleft licences and copyright law, permissive free software licences do not control the licence terms that a derivative work falls under. Nevertheless, the quote describes the permissive licence users' unconcern for the discussion on freedoms.
Some permissive free software licences contain clauses that make them incompatible with copyleft licences. One example is clauses requiring advertising materials to credit the copyright holder. Licences with this type of advertising clause include the 4-clause BSD licence, the PHP License, and the OpenSSL Licence.
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.
Some licences do not allow derived works to add a restriction that says a redistributor cannot add more restrictions. Examples include the CDDL and MsPL. However such restrictions also make the licence incompatible with permissive free software licences.