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A free-software license is a notice that grants the recipient of a piece of software extensive rights to modify and redistribute that software. These actions are usually prohibited by
copyright Copyright is a type of intellectual property Intellectual property (IP) is a category of property Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. ...

copyright
law, but the rights-holder (usually the author) of a piece of software can remove these restrictions by accompanying the software with a
software license A software license is a legal instrument (usually by way of contract law A contract is a legally binding agreement that defines and governs the rights and duties between or among its parties Image:'Hip, Hip, Hurrah! Artist Festival at Sk ...
which grants the recipient these rights. Software using such a license is free software (or
free and open-source software Free and open-source software (FOSS) is software Software is a collection of instructions that tell a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. ...
) as conferred by the copyright holder. Free-software licenses are applied to
software Software is a collection of instructions Instruction or instructions may refer to: Computing * Instruction, one operation of a processor within a computer architecture instruction set * Computer program, a collection of instructions Music * I ...

software
in
source code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and ...

source code
and also binary object-code form, as the copyright law recognizes both forms.


Comparison

Free-software licenses provide risk mitigation against different legal threats or behaviors that are seen as potentially harmful by developers:


History


Pre-1980s

In the early times of software, sharing of software and source code was common in certain communities, for instance academic institutions. Before the US Commission on New Technological Uses of Copyrighted Works (CONTU) decided in 1974 that "computer programs, to the extent that they embody an author's original creation, are proper subject matter of copyright",Lemley, Menell, Merges and Samuelson. ''Software and Internet Law'', p. 34. software was not considered copyrightable. Therefore, software had no licenses attached and was shared as
public-domain software Public-domain software is software Software is a collection of Instruction (computer science), instructions and data (computing), data that tell a computer how to work. This is in contrast to Computer hardware, physical hardware, from which the ...
. The CONTU decision plus court decisions such as ''
Apple v. Franklin ''Apple Computer, Inc. v. Franklin Computer Corp.'', 714 F.2d 1240 ( 3d Cir. 1983), was the first time an appellate level court in the United States The United States of America (USA), commonly known as the United States (U.S. or US), or Am ...
'' in 1983 for
object code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...
, clarified that the Copyright Act gave computer programs the copyright status of literary works and started the licensing of software. Free-software licenses before the late 1980s were generally informal notices written by the developers themselves. These early licenses were of the "
permissive{{about, , the 1970 British film, Permissive (film), the grammatical mode, Permissive mood, the flavor of software license, permissive free software licence A permissive cell or host is one that allows a virus A virus is a submicroscopic i ...
" kind.


1980s

In the mid-1980s, the
GNU project The GNU Project () is a free software Free software (or libre software) is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any adapted versi ...
produced
copyleft Copyleft is the practice of granting the right to freely distribute and modify with the requirement that the same rights be preserved in derivative works created from that property. Copyleft in the form of s can be used to maintain conditions ...

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 GNU Emacs is a free software text editor. It was created by GNU Project founder Richard Stallman. In common with other varieties of Emacs, GNU Emacs is extensible using a Turing complete programming language. GNU Emacs has been called "the most p ...
in 1985, which was revised into the "GNU Emacs General Public License" in late 1985, and clarified in March 1987 and February 1988. Likewise, the similar GCC General Public License was applied to the
GNU Compiler Collection The GNU Compiler Collection (GCC) is an optimizing compiler In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an ...
, which was initially published in 1987. The original BSD license is also one of the first free-software licenses, dating to 1988. In 1989, version1 of the
GNU General Public License The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software license A free-software license is a notice that grants the recipient of a piece of software extensive rights to modify and software distribu ...
(GPL) was published. Version2 of the GPL, released in 1991, went on to become the most widely used free-software license.


1990s to 2000s

Starting in the mid-1990s and until the mid-2000s, the
open-source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized softwar ...
movement pushed and focused the free-software idea forward in the wider public and business perception. In the
Dot-com bubble The dot-com bubble, also known as the dot-com boom, the tech bubble, and the Internet bubble, was a stock market bubble Stock (also capital stock) is all of the Share (finance), shares into which ownership of a corporation is divided.Longm ...
time,
Netscape Communications Netscape Communications Corporation (originally Mosaic Communications Corporation) was an American independent computer services company with headquarters in Mountain View, California Mountain View is a city in Santa Clara County, California ...
' step to release its webbrowser under a FOSS license in 1998, inspired many other companies to adapt to the FOSS ecosystem. In this trend companies and new projects (
Mozilla Mozilla (stylized as moz://a) is a free software Free software (or libre software) is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any a ...

Mozilla
,
Apache foundation The Apache Software Foundation (ASF) is an American nonprofit corporation (classified as a 501(c)(3) organization in the United States) to support Apache software projects, including the Apache HTTP Server. The ASF was formed from the Apache Group ...
, and
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...
, see also this list) wrote their own FOSS licenses, or adapted existing licenses. This
License proliferation License proliferation is the phenomenon of an abundance of already existing and the continued creation of new software license A software license is a legal instrument (usually by way of contract law, with or without printed material) governin ...
was later recognized as problem for the
Free and open-source Free and open-source software (FOSS) is software Software is a collection of instructions Instruction or instructions may refer to: Computing * Instruction, one operation of a processor within a computer architecture instruction set * Com ...
ecosystem due to the increased complexity of
license compatibility 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 ...
considerations. While the creation of new licenses slowed down later, license proliferation and its impact are considered an ongoing serious challenge for the free and open-source ecosystem. From the free-software licenses, the GNU GPL version2 has been tested in to court, first in Germany in 2004 and later in the USA. In the German case the judge did not explicitly discuss the validity of the GPL's clauses but accepted that the GPL had to be adhered to: "If the GPL were not agreed upon by the parties, defendant would notwithstanding lack the necessary rights to copy, distribute, and make the software 'netfilter/iptables' publicly available." Because the defendant did not comply with the GPL, it had to cease use of the software. The US case (
MySQL MySQL () is an open-source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source m ...
vs Progress) was settled before a verdict was arrived at, but at an initial hearing, Judge Saris "saw no reason" that the GPL would not be enforceable. Around 2004 lawyer Lawrence Rosen argued in the essay ''Why the public domain isn't a license'' software could not truly be waived into public domain and can't be interpreted as very permissive FOSS license, a position which faced opposition by Daniel J. Bernstein and others. In 2012 the dispute was finally resolved when Rosen accepted the
CC0 A Creative Commons (CC) license is one of several public copyright licenses that enable the free distribution of an otherwise copyrighted "work".A "work" is any creative material made by a person. A painting, a graphic, a book, a song/lyrics t ...

CC0
as
open source license An open-source license is a type of license A license (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to ...
, while admitting that contrary to his previous claims copyright can be waived away, backed by
Ninth circuit The United States Court of Appeals for the Ninth Circuit (in case citations, 9th Cir.) is a federal court of appeals that has appellate jurisdiction Appellate jurisdiction is the power of an appellate court to review, amend and overrule decis ...
decisions. In 2007, after years of draft discussion, the GPLv3 as major update of the GPLv2 was released. The release was controversial due to the significant extended scope of the license, which made it incompatible with the GPLv2. Several major FOSS projects (
Linux kernel The Linux kernel is a free and open-source Free and open-source software (FOSS) is software that is both free software and open-source software where anyone is free software license, freely licensed to use, copy, study, and change the softwar ...
,
MySQL MySQL () is an open-source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source m ...
,
BusyBox BusyBox is a software suite that provides several Unix utilities in a single executable file. It runs in a variety of POSIX The Portable Operating System Interface (POSIX) is a family of standardization, standards specified by the IEEE Comp ...
,
Blender A blender (sometimes called a mixer or liquidiser in British English British English (BrE) is the standard dialect of the English language English is a West Germanic languages, West Germanic language first spoken in History of Ang ...
, VLC media player) decided against adopting the GPLv3. On the other hand, in 2009, two years after the release of the GPLv3,
Google Google LLC is an American multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Multinational stat ...

Google
open-source programs office manager Chris DiBona reported that the number of open-source projects licensed software that had moved to GPLv3 from GPLv2 was 50%, counting the projects hosted at
Google Code Google Developers (previously Google Code) , s (APIs), and technical resources. The site contains documentation on using Google developer tools and APIs—including discussion groups and blogs for developers using Google's developer products. T ...
.


2010s

In 2011, four years after the release of the GPLv3, 6.5% of all open-source licensed projects were GPLv3 while 42.5% were still GPLv2 according to Black Duck Software data. Following in 2011 ''451 Group'' analyst Matthew Aslett argued in a blog post that copyleft licenses went into decline and permissive licenses increased, based on statistics from Black Duck Software. In 2015 according to Black Duck Software and
GitHub __FORCETOC__ GitHub, Inc. is a provider of Internet hosting service, Internet hosting for software development and version control using Git. It offers the distributed version control and source code management (SCM) functionality of Git, plus ...

GitHub
statistics, the permissive MIT license dethroned the GPLv2 as most popular free-software license to the second place while the permissive Apache license follows already at third place. In June 2016 an analysis of
Fedora Project The Fedora Project is an independent project to co-ordinate the development of Fedora Linux, a Linux kernel-based operating system, operating with the vision of "''a world where everyone benefits from free and open source software built by inclus ...
's packages revealed as most used licenses the GPL, MIT, BSD, and the
LGPL The GNU Lesser General Public License (LGPL) is a free-software license published by the Free Software Foundation (FSF). The license allows developers and companies to use and integrate a software component released under the LGPL into their own ...
.


Definitions


OSI-approved open-source licenses

The group
Open Source Initiative The Open Source Initiative (OSI) is the steward of the Open Source definition, Open Source Definition, the set of rules that define Open Source Software, open source software. It is a California public-benefit nonprofit corporation, with 501(c)(3 ...
(OSI) defines and maintains a list of approved
open-source license An open-source license is a type of License compatibility, license for computer software and other products that allows the source code, blueprint or design to be used, modified and/or shared under defined terms and conditions. This allows end user ...
s. OSI agrees with FSF on all widely used free-software licenses, but differ from FSF's list, as it approves against the
Open Source Definition ''The Open Source Definition'' is a document published by the Open Source Initiative, to determine whether a software license can be labeled with the Open-source software, open-source certification mark. The definition was taken from the exact te ...
rather than the
Free Software Definition The Free Software Definition written by Richard Stallman Richard Matthew Stallman (; born March 16, 1953), also known by his initials, rms, is an American free software movement activist and programmer A computer programmer, sometimes cal ...
. It considers Free Software Permissive license group to be a reference implementation of a Free Software license. Thus its requirements for approving licenses are different.


FSF-approved free-software licenses

The
Free Software Foundation The Free Software Foundation (FSF) is a 501(c)(3) A 501(c)(3) organization is a corporation, trust, unincorporated association, or other type of organization exempt from federal income tax under section 501(c)(3) of Title 26 of the United States ...
, the group that maintains the
Free Software Definition The Free Software Definition written by Richard Stallman Richard Matthew Stallman (; born March 16, 1953), also known by his initials, rms, is an American free software movement activist and programmer A computer programmer, sometimes cal ...
, maintains a non-exhaustive list of free-software licences. The Free Software Foundation prefers
copyleft Copyleft is the practice of granting the right to freely distribute and modify with the requirement that the same rights be preserved in derivative works created from that property. Copyleft in the form of s can be used to maintain conditions ...

copyleft
(
share-alike The copyleft.html"_;"title="Creative_Commons_icon_for_Share-Alike,_a_variant_of_the_copyleft">Creative_Commons_icon_for_Share-Alike,_a_variant_of_the_copyleft_symbol Share-alike_is_a_Copyright.html" ;"title="copyleft_symbol.html" ;"title="copylef ...
) 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 The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software license A free-software license is a notice that grants the recipient of a piece of software extensive rights to modify and software distribu ...
.


Conditions in free-software licenses

There exists an ongoing debate within the free-software community regarding the fine line between what restrictions can be applied and still be called "free". Only "
public-domain software Public-domain software is software Software is a collection of Instruction (computer science), instructions and data (computing), data that tell a computer how to work. This is in contrast to Computer hardware, physical hardware, from which the ...
" and software under a public-domain-like license is restriction-free. Examples of public-domain-like licenses are, for instance, the
WTFPL WTFPL is a permissive{{about, , the 1970 British film, Permissive (film), the grammatical mode, Permissive mood, the flavor of software license, permissive free software licence A permissive cell or host is one that allows a virus A viru ...
and the
CC0 A Creative Commons (CC) license is one of several public copyright licenses that enable the free distribution of an otherwise copyrighted "work".A "work" is any creative material made by a person. A painting, a graphic, a book, a song/lyrics t ...

CC0
license.
Permissive license A permissive software license, sometimes also called BSD-like or BSD-style license, is a free-software Free software (or libre software) is computer software Software is a collection of Instruction (computer science), instructions and ...
s might carry small obligations like
attribution Attribution may refer to: * Attribution (copyright), concept in copyright law requiring an author to be credited * Attribution (journalism), the identification of the source of reported information * Attribution (law), legal doctrines by which lia ...
of the author but allow practically all code use cases. Certain licenses, namely the
copyleft license Copyleft is the practice of granting the right to freely distribute and modify intellectual property Intellectual property (IP) is a category of property that includes intangible creations of the human intellect. There are many types of inte ...
s, include intentionally stronger restrictions (especially on the distribution/distributor) in order to force derived projects to guarantee specific rights which can't be taken away.


Copyleft

The free-software share-alike licenses written by
Richard Stallman Richard Matthew Stallman (; born March 16, 1953), also known by his initials, rms, is an American free software movement The free software movement is a social movement A social movement is a loosely organized effort by a large group of peo ...

Richard Stallman
in the mid-1980s pioneered a concept known as "copyleft". Ensuing copyleft provisions stated that when modified versions of free software are distributed, they must be distributed under the same terms as the original software. Hence they are referred to as "share and share alike" or "''quid pro quo''". This results in the new software being open source as well. Since copyleft ensures that later generations of the software grant the freedom to modify the code, this is "free software". Non-copyleft licenses do not ensure that later generations of the software will remain free. Developers who use GPL code in their product must make the
source code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and ...

source code
available to anyone when they share or sell the
object code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...
. In this case, the source code must also contain any changes the developers may have made. If GPL code is used but not shared or sold, the code is not required to be made available and any changes may remain private. This permits developers and organizations to use and modify GPL code for private purposes (that is, when the code or the project is not sold or otherwise shared) without being required to make their changes available to the public. Supporters of GPL claim that by mandating that derivative works remain under the GPL, it fosters the growth of free software and requires equal participation by all users. Opponents of GPL claim that "no license can guarantee future software availability" and that the disadvantages of GPL outweigh its advantages. Some also argue that restricting distribution makes the license less free. Whereas proponents would argue that not preserving freedom during distribution would make it less free. For example, a non-copyleft license does not grant the author the freedom to see modified versions of his or her work if it gets publicly published, whereas a copyleft license does grant that freedom.


Patent retaliation

During the 1990s, free-software licenses began including clauses, such as patent retaliation, in order to protect against
software patent A software patent is a patent NPOV disputes from March 2021 A patent is a Title (property), title that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of years in exchange fo ...
litigation cases – a problem which had not previously existed. This new threat was one of the reasons for writing version3 of the GNU GPL in 2006. In recent years, a term coined
tivoization Tivoization is the creation of a system that incorporates software under the terms of a copyleft software license like the GNU General Public License (GNU GPL), but uses hardware restrictions or digital rights management (DRM) to prevent users fr ...
describes a process where hardware restrictions are used to prevent users from running modified versions of the software on that hardware, in which the
TiVo TiVo ( ) is a digital video recorder A digital video recorder (DVR) is an electronic device that records video in a digital format to a disk drive Disk storage (also sometimes called drive storage) is a general category of storage mechanisms w ...
device is an example. It is viewed by the FSF as a way to turn free software to effectively non-free, and is why they have chosen to prohibit it in
GPLv3 The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end users the Four Freedoms (Free software), four freedoms to run, study, share, and modify the software. The licenses were ...
. Most newly written free-software licenses since the late 1990s include some form of patent retaliation clauses. These measures stipulate that one's rights under the license (such as to redistribution), may be terminated if one attempts to enforce patents relating to the licensed software, under certain circumstances. As an example, the
Apple Public Source License The Apple Public Source License (APSL) is the open-source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of ...
may terminate a user's rights if said user embarks on litigation proceedings against them due to patent litigation. Patent retaliation emerged in response to proliferation and abuse of
software patents A software patent is a patent NPOV disputes from March 2021 A patent is a Title (property), title that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of years in exchange fo ...
.


Hardware restrictions

Version3 of the GNU GPL includes specific language prohibiting additional restrictions being enforced by
hardware restrictions A hardware restriction (sometimes called hardware DRM) is content protection enforced by electronic component An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their as ...
and digital rights management (DRM), a practice FSF calls
tivoization Tivoization is the creation of a system that incorporates software under the terms of a copyleft software license like the GNU General Public License (GNU GPL), but uses hardware restrictions or digital rights management (DRM) to prevent users fr ...
after
Tivo TiVo ( ) is a digital video recorder A digital video recorder (DVR) is an electronic device that records video in a digital format to a disk drive Disk storage (also sometimes called drive storage) is a general category of storage mechanisms w ...
used GPL’d software on devices that disallowed user modification of that software.


Attribution, disclaimers and notices

The majority of free-software licenses require that modified software not claim to be unmodified. Some licenses also require that copyright holders be credited. One such example is version2 of the GNU GPL, which requires that interactive programs that print warranty or license information, may not have these notices removed from modified versions intended for distribution.


Practical problems with licenses


License compatibility

Licenses of software packages containing contradictory requirements render it impossible to combine source code from such packages in order to create new software packages. License compatibility between a
copyleft license Copyleft is the practice of granting the right to freely distribute and modify intellectual property Intellectual property (IP) is a category of property that includes intangible creations of the human intellect. There are many types of inte ...
and another license is often only a one-way compatibility. This "one-way compatibility" characteristic is, for instanced, criticized by the
Apache Foundation The Apache Software Foundation (ASF) is an American nonprofit corporation (classified as a 501(c)(3) organization in the United States) to support Apache software projects, including the Apache HTTP Server. The ASF was formed from the Apache Group ...
, who provides the more
permissive{{about, , the 1970 British film, Permissive (film), the grammatical mode, Permissive mood, the flavor of software license, permissive free software licence A permissive cell or host is one that allows a virus A virus is a submicroscopic i ...
Apache license which doesn't have this characteristic. Non-copyleft licenses, such as the FOSS
permissive licenses A permissive software license, sometimes also called BSD-like or BSD-style license, is a free-software license with only minimal restrictions on how the software can be used, modified, and redistributed, usually including a warranty disclaimer. ...
, have a less complicated license interaction and normally exhibit better license compatibility. For example, if one license says "modified versions must mention the developers in any advertising materials", and another license says "modified versions cannot contain additional attribution requirements", then, if someone combined a software package which uses one license with a software package which uses the other, it would be impossible to distribute the combination because these contradictory requirements cannot be fulfilled simultaneously. Thus, these two packages would be license-incompatible. When it comes to
copyleft Copyleft is the practice of granting the right to freely distribute and modify with the requirement that the same rights be preserved in derivative works created from that property. Copyleft in the form of s can be used to maintain conditions ...

copyleft
software licenses, they are not inherently compatible with other copyleft licenses, even the GPLv2 is, by itself, not compatible with the GPLv3.


Purpose of use

Restrictions on use of a software ("use restrictions") are generally unacceptable according to the FSF, OSI,
Debian Debian (), also known as Debian GNU/Linux, is a GNU/Linux distribution composed of free and open-source software, developed by the community-supported Debian Project, which was established by Ian Murdock on August 16, 1993. The first version of ...

Debian
, or the BSD-based distributions. Examples include prohibiting that the software be used for non-private applications, for military purposes, for comparison or benchmarking, for good use, for ethically questionable means, or in commercial organizations. While some restrictions on user freedom, e.g. concerning nuclear war, seem to enjoy moral support among most free software developers, it is generally believed that such agendas should not be served through software licenses; among other things because of practical aspects such as resulting legal uncertainties and problems with enforceability of vague, broad and/or subjective criteria or because tool makers are generally not held responsible for other people’s use of their tools. Nevertheless some projects include legally non-binding pleas to the user, prominently
SQLite SQLite (, ) is a (RDBMS) contained in a . In contrast to many other , SQLite is not a database engine. Rather, it is embedded into the end program. SQLite generally follows syntax. SQLite uses a dynamically and that does not guarantee ...
. Among the repeated attempts by developers to regulate user behavior through the license that sparked wider debate are
Douglas Crockford Douglas Crockford is an United States, American computer programmer and entrepreneur who is involved in the development of the JavaScript language. He popularized the data format JSON (JavaScript Object Notation), and has developed various JavaScr ...

Douglas Crockford
’s (joking) “no evil” clause, which affected the release process of the Debian distribution in 2012 and got the JSMin-PHP project expelled from
Google Code Google Developers (previously Google Code) , s (APIs), and technical resources. The site contains documentation on using Google developer tools and APIs—including discussion groups and blogs for developers using Google's developer products. T ...
, the addition of a pacifist condition based on Asimov’s First Law of Robotics to the GPL for the distributed computing software ''GPU'' in 2005, as well as several software projects trying to exclude use by big cloud providers.


Definition conflicts

As there are several defining organizations and groups who publish definitions and guidelines about FOSS licenses, notably the FSF, the OSI, the Debian project, and the BSDs, there are sometimes conflicting opinions and interpretations.


Permissive versus copyleft opinions

Many users and developers of
BSD The Berkeley Software Distribution (BSD) is a discontinued based on , developed and distributed by the (CSRG) at the . The term "BSD" commonly refers to its descendants, including , , , and . BSD was initially called Berkeley Unix because it ...
-based operating systems have a different position on licensing. The main difference is the belief that the
copyleft Copyleft is the practice of granting the right to freely distribute and modify with the requirement that the same rights be preserved in derivative works created from that property. Copyleft in the form of s can be used to maintain conditions ...

copyleft
licenses, particularly the
GNU General Public License The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software license A free-software license is a notice that grants the recipient of a piece of software extensive rights to modify and software distribu ...
(GPL), are undesirably complicated and/or restrictive. The GPL requires any derivative work to also be released according to the GPL while the
BSD license BSD licenses are a family of permissive free software licenses, imposing minimal restrictions on the use and distribution of covered software. This is in contrast to copyleft Copyleft is the practice of granting the right to freely distrib ...
does not. Essentially, the BSD license's only requirement is to acknowledge the original authors, and poses no restrictions on how the
source code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and ...

source code
may be used. As a result, BSD code can be used in
proprietary software Proprietary software, also known as non-free software or closed-source software, is computer software for which the software's publisher or another person reserves some rights from licenses to use, modify, share modifications, or share the softwa ...
that only acknowledges the authors. For instance, Microsoft Windows NT 3.1 and
macOS macOS (; previously Mac OS X and later OS X) is a proprietary {{Short pages monitor