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The Free Software Foundation prefers copyleft (share-alike) free-software licensing rather than permissive free-software licensing for most purposes. Its list distinguishes between free-software licenses that are compatible or incompatible with the FSF's copyleft GNU General Public License .
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.
The two main categories of free and open-source licenses are permissive and copyleft. [14] Both grant permission to change and distribute software. Typically, they require attribution and disclaim liability. [15] [16] Permissive licenses come from academia. [17] Copyleft licenses come from the free software movement. [18]
There are licenses accepted by the OSI which are not free as per the Free Software Definition. The Open Source Definition allows for further restrictions like price, type of contribution and origin of the contribution, e.g. the case of the NASA Open Source Agreement, which requires the code to be "original" work.
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.
Free-software licenses that use "weak" copyleft include the GNU Lesser General Public License and the Mozilla Public License. The GNU General Public License is an example of a license implementing strong copyleft. An even stronger copyleft license is the AGPL, which requires the publishing of the source code for software as a service use cases.
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 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.