Editing Open-source license (section)

===Copyleft===
{{main|Copyleft}}
[[File:"Copyleft (L)" sticker, from an envelope mailed from Don Hopkins to Richard Stallman in 1984..jpg|alt=A sticker reads, "Copyleft circled letter L".|thumb|The Copyleft sticker from an envelope Don Hopkins mailed to Richard Stallman in 1984]]

[[Copyleft]] licenses require [[source code]] to be distributed with software and require the source code to be made available under a similar license.{{sfn|Sen|Subramaniam|Nelson|2008|pp=211-212}}{{sfn|St. Laurent|2004|pp=38-39}} Like the permissive licenses, most copyleft licenses require attribution.{{sfn|Ballhausen|2019|p=86}} Most, including the GPL, disclaim implied warranties.{{sfn|Rosen|2005|p=135}}

Copyleft uses the restrictions of IP law&mdash;contrary to their usual purpose&mdash;to mandate that the code remain open.{{sfn|Rosen|2005|pp=103-106}} The term and its related slogan, "All rights reversed", had been previously used in a playful manner by the ''[[Principia Discordia]]'' and [[Tiny BASIC]]; the modern usage begins with Richard Stallman's efforts to create a free operating system. In 1984, programmer [[Don Hopkins]] mailed a manual to Stallman with a "Copyleft Ⓛ" sticker. Stallman, who was working on the GNU operating system, adopted the term.{{sfn|Keats|2010|p=64}} An early version of copyleft licensing was used for the 1985 release of [[GNU Emacs]].{{sfn|Williams|2002|loc=ch.&nbsp;9}}<ref>{{cite web |url=https://www.tech-insider.org/free-software/research/1985/0715-b.txt |title=Full Text of GNU Emacs Copying Permission Notice |date=1985}}</ref> The term became associated with the FSF's later reciprocal licenses, notably the [[GNU General Public License]] (GPL).{{sfn|Keats|2010|pp=63-67}}

Traditional, proprietary software licenses are written with the goal of increasing [[Software industry|profit]], but Stallman wrote the GPL to increase the body of available free software. His reciprocal licenses offer the rights to use, modify, and distribute the work on the condition that people must release derivative works under a license offering these same freedoms. Software built on a copyleft base must come with the source code, and the source code must be available under the same or a similar license. This offers protection against proprietary software consuming code without giving back.{{sfn|Rosen|2005|pp=103-109}}{{sfn|Meeker|2020|loc=6:00–7:22}} Richard Stallman stated that "the central idea of copyleft is to use copyright law, but flip it over to serve the opposite of its usual purpose: instead of a means of privatizing software, [copyright] becomes a means of keeping software free."{{sfn|Joy|2022|pp=990-992}} [[Free software licenses]] are also open-source software licenses.{{sfn|Onetti|Verma|2009|p=71}} The separate terms [[free software]] and [[open-source software]] reflect different values rather than a legal difference.{{sfn|St. Laurent|2004|pp=81-83, 114}} Both movements and their formal definitions require the covered work to be made available with source code and with permission for modification and redistribution.{{sfn|Maracke|2019|loc=sec. 2.2}} There are occasional edge cases where only one of the FSF or the OSI accept a license, but the popular free software licenses are open source, including the [[GPL]].{{sfn|Ballhausen|2019|p=82}}

[[File:Mitchell Baker (2).jpg|alt=Portrait of Mitchell Baker|thumb|[[Mitchell Baker]] drafted the Mozilla Public License while on Netscape's legal team.{{sfn|St. Laurent|2004|pp=68, 75}}]]

Practical benefits to copyleft licenses have attracted commercial developers. Corporations have used and written reciprocal licenses with a narrower scope than the GPL.{{sfn|Tsai|2008|pp=564-570}} For example, Netscape drafted their own copyleft terms after rejecting permissive licenses for the Mozilla project.{{sfn|Hammerly|Paquin|Walton|1999}}  The GPL remains the most popular license of this type, but there are other significant examples. The FSF has crafted the [[Lesser General Public License]] (LGPL) for [[Library (computing)|libraries]]. [[Mozilla]] uses the [[Mozilla Public License]] (MPL) for their releases, including [[Firefox]]. [[IBM]] drafted the [[Common Public License]] (CPL) and later adopted the [[Eclipse Public License]] (EPL). A difference between the GPL and other reciprocal licenses is how they define derivative works covered by the reciprocal provisions. The GPL, and the [[Affero License]] (AGPL) based on it, use a broad scope to describe affected works. The AGPL extends the reciprocal obligation in the GPL to cover software made available over a network.{{sfn|Tsai|2008|pp=564-570}}{{sfn|Hammerly|Paquin|Walton|1999}} They are called [[Copyleft#Strong and weak copyleft|strong copyleft]] in contrast to the [[Copyleft#Strong and weak copyleft|weaker copyleft]] licenses often used by corporations. Weak copyleft uses narrower, explicit definitions of derivative works.{{sfn|Sen|Subramaniam|Nelson|2008|pp=212-213}}{{sfn|Meeker|2020|loc=16:13}} The MPL uses a file-based definition, the CPL and EPL use a module-based definition, and the FSF's own LGPL refers to software libraries.{{sfn|Rosen|2005|loc=refer to corresponding chapters}}