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Copyleft is a distinguishing feature of some free software licenses, while other free-software licenses are not copyleft licenses because they do not require the licensee to distribute derivative works under the same license. There is an ongoing debate as to which class of license provides the greater degree of freedom.
A permissive software license, sometimes also called BSD-like or BSD-style license, [1] is a free-software license which instead of copyleft protections, carries only minimal restrictions on how the software can be used, modified, and redistributed, usually including a warranty disclaimer.
Permissive licenses come from academia. Copyleft licenses come from the free software movement. Copyleft licenses require derivative works to be distributed with the source code and under a similar license. Since the mid-2000s, courts in multiple countries have upheld the terms of both types of license.
License compatibility is a legal framework that allows for pieces of software with different software licenses to be distributed together. The need for such a framework arises because the different licenses can contain contradictory requirements, rendering it impossible to legally combine source code from separately-licensed software in order to create and publish a new program.
Permissive licenses allow for unrestricted use, modification, and distribution of a copyrighted work. The modified BSD license, the X11 license, and the MIT license are each examples of permissive licenses. These licenses seek to make it as easy as possible to reuse the licensed work: the objective is generally to make the work available and as ...
In the mid-1980s, the GNU project produced copyleft free-software licenses for each of its software packages. An early such license (the "GNU Emacs Copying Permission Notice") was used for GNU Emacs in 1985, [5] which was revised into the "GNU Emacs General Public License" in late 1985, and clarified in March 1987 and February 1988.
Copyleft is a type of free license that mandates derivative works to be licensed. The other types of free license lack this requirement: for permissive licenses, attribution is typically the only requirement, and public-domain-equivalent licenses have no restrictions.
The licenses in the GPL series are all copyleft licenses, which means that any derivative work must be distributed under the same or equivalent license terms. It is more restrictive than the Lesser General Public License, and even further distinct from the more widely-used permissive software licenses such as BSD, MIT, and Apache.