Open-source license

Permissive licenses, also known as academic licenses, allow recipients to use, modify, and distribute software with no obligation to provide source code. Institutions created these licenses to distribute their creations to the public.[21] Permissive licenses are usually short, often less than a page of text. They impose few conditions. Most include disclaimers of warranty and obligations to credit authors. A few include explicit provisions for patents, trademarks, and other forms of intellectual property.[22]

The University of California, Berkeley created the first open-source license, the Berkeley Software Distribution (BSD) license to permit free usage with no obligations placed on users. The BSD licenses brought the concept of academic freedom of ideas to computing. Early academic software authors had shared code based on implied promises. Berkley made these concepts explicit with clear disclaimers for liability and warranty along with conditions, or clauses, for redistribution. The original had 4 clauses but subsequent versions have further reduced the restrictions. As a result, it's common to specify if software uses a 2-clause or 3-clause version.[23]

The Massachusetts Institute of Technology (MIT) created an academic license based on the BSD original. The MIT license clarified the conditions by making them more explicit. For example, the MIT license describes the right to sublicense. One of the strengths of open-source development is the complex process where developers can build on the derivative works of each other and combine their projects into collective works. Explicitly making covered code sublicensable provides a legal advantage when tracking the chain of authorship.[24] The BSD and MIT are template licenses that can be adapted to any project. They are widely adapted and used by many FOSS projects.[23]

The Apache License is more comprehensive and explicit. The Apache Software Foundation wrote it for their Apache HTTP Server. Version 2, published in 2004, offers legal advantages over simple licenses and provides similar grants.[25] While the BSD and MIT licenses offer an implicit patent grant, the Apache License includes a section on patents with an explicit grant from contributors. Additionally, it is one of the few permissive licenses with a patent retaliation clause. Patent retaliation, or patent suspension, clauses take effect if a licensee initiates patent infringement litigation on covered code. In that situation, the patent grants are revoked. These clauses protect against patent trolling.[17]

The Copyleft sticker from an envelope Don Hopkins mailed to Richard Stallman in 1984.

Copyleft uses reciprocity to subvert restrictions in IP law. Copyleft originates in the free software movement, science fiction fandom, and the broader counterculture. According to the 1910 Buenos Aires Convention, copyrighted literature needed a reservation of rights, or copyright notice. These were typically a line of text reading "Copyright © All rights reserved." These copyright formalities would be printed on the title page of a book or somewhere on the packaging of other media. In the Discordian satirical but sacred book, the Principia Discordia, Gregory Hill used instead a notice that read, "Ⓚ All rights reversed." The phrase "all rights reversed" entered into the computing and science fiction culture as expressions of resistance to IP law and its restrictions. It was often paired with "copyleft" and playful alternatives to the copyright symbol. In 1984, programmer Don Hopkins mailed a manual to Richard Stallman with a "Copyleft Ⓛ" sticker. Stallman was working on the GNU project at that time and adopted the term to describe his goal of reciprocity. The GNU project employed an early version of copyleft with the 1985 release of GNU Emacs.[26] The term would become associated with the FSF's later reciprocal licenses, notably the GNU General Public License (GPL). The FSF popularized the word and associated it with reciprocal free software.[27]

Traditional, proprietary software licenses are written with the goal of increasing 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. This offers protection against proprietary software consuming code without giving back.[28] 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."[29]

Courts have found that distributing copyleft applications indicates acceptance of the terms.[30] Physical software releases can acquire the consumer's assent with notices placed on shrinkwrap. Online distribution can use clickwrap, a digital equivalent where the user must click to accept. Copyleft releases have an additional acceptance mechanism. Without permission from the copyright holder, the law prohibits redistribution, public displays, and the release of modified versions. Therefore, engaging in these actions as allowed by an open-source license is treated as an indication of acceptance of the requirement to provide source code under the appropriate terms.[31]

Practical benefits to copyleft licenses have attracted commercial developers. Corporations have used and written reciprocal licenses with a narrower scope than the GPL. 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 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.[32] They are called strong copyleft in contrast to the weaker copyleft licenses often used by corporations. Weak copyleft uses narrower, explicit definitions of derivative works.[33] The MPL uses a file-based definition, the CPL/EPL use a module-based definition, and FSF's own LGPL exempts libraries in certain situations.[34]

License compatibility determines how code with different licenses can be distributed together. The goal of open-source licensing is to make the work freely available, but this becomes complicated when working with multiple terminologies imposing different requirements.[35] There are many uncommonly used licenses and some projects write their own bespoke agreements. As a result, this causes more confusion than other legal aspects. When releasing a collection of applications, each license can be considered separately. However, when attempting to combine software, code from another project can only be in-licensed if the project uses compatible terms and conditions.[36]

When combining code bases, the original licenses can be maintained for separate components, and the larger work released under a compatible license. Permissive licenses are broadly compatible because they can cover separate parts of a project. The GPL, LGPL, AGPL, MPL, EPL, and Apache License have all been revised to enhance compatibility.[37] This compatibility is often one-way. Permissive licenses can be used within copyleft works, but copyleft material cannot be released under a permissive license. Some weak copyleft licenses can be used under the GPL and are said to be GPL-compatible. GPL software can only be used under the GPL or AGPL. Public domain content can be used anywhere as there is no copyright claim, but code acquired under any almost any set of terms cannot be waved to the public domain. [35]

Free software licenses are also open-source software licenses. The separate terms free software and open-source software reflect different values rather than a legal difference. The founder of the FSF, Richard Stallman, stated that "free software is an ethical imperative" in contrast to the practical aims of open source.[38] Bruce Perens based the Open Source Definition on the Debian project's guidelines which were based on Stallman's Free Software Definition.[39]

There are occasional edge cases where only one of the FSF or the OSI accept a license. For example, only the OSI approved the Open Watcom license. The FSF viewed their Sybase Open Watcom Public License as non-free because it required published source code for private modifications. Situations like this are rare, and the popular free software licenses are open source, including the GPL.[40]

Early computer programs like the pioneering video game Spacewar! are in the public domain.[41]

When a copyright expires, the work enters the public domain, and is freely available to anyone.[42] Some creative works are not covered by copyright and enter directly into the public domain. In the early history of computing, this applied to software. Some early pieces of computer software were published without copyright protections.[43] According to attorney Lawrence Rosen, copyright laws were not written with the expectation that creators would place their work into the public domain. Thus intellectual property laws lack clear paths to waive a copyright. Highly permissive licenses described as "public domain" may legally function as unilateral contracts that offer something but impose no terms.[44]

A public-domain-equivalent license, like the Creative Commons CC0, provides a waiver of copyright claims into the public domain along with a permissive software license as a fallback. In jurisdictions that do not accept a public domain waiver, the permissive license takes effect.[45] Public domain waivers share limitations with simple academic licenses. This creates the possibility that an outside party could attempt to control a public domain work via patent or trademark law.[46] Public domain waivers handle warranties differently from any type of license. Even very permissive ones, like the MIT license, disclaim warranty and liability. Anyone using the free software must accept this disclaimer as a condition. Because public domain content is available to everyone, the copyright waiver cannot impose a disclaimer.[42]

FOSS is often free of cost. Freeware is free of cost, but the term usually describes gratis proprietary software.

Freeware is software distributed at no cost. FOSS is often given away gratis, but "freeware" typically refers to proprietary software, which does not grant permission to modify or redistribute copies. Proprietary freeware licenses limit redistribution, prohibit commercial usage, limit installations,[47] and often include an end-user license agreement. Many proprietary software companies distribute freeware, including Microsoft who was the world's largest supplier in 2014.[48] Source-available software is proprietary freeware that comes with source code as a reference.[49]