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Copyleft (a play on the word ''copyright'') is the practice of offering people the right to freely distribute copies and modified versions of a work with the stipulation that the same rights be preserved in derivative works down the line.〔(【引用サイトリンク】title=What is Copyleft? )〕 Copyleft is a form of licensing, and can be used to maintain copyright conditions for works ranging from computer software, to documents, to art. In general, copyright law is used by an author to prohibit recipients from reproducing, adapting, or distributing copies of their work. In contrast, under copyleft, an author may give every person who receives a copy of the work permission to reproduce, adapt, or distribute it, with the accompanying requirement that any resulting copies or adaptations are also bound by the same licensing agreement. Copyleft licenses for software require that information necessary for reproducing and modifying the work must be made available to recipients of the binaries. The source code files will usually contain a copy of the license terms and acknowledge the authors. Copyleft type licenses are a novel use of existing copyright law to ensure a work remains freely available. The GNU General Public License, originally written by Richard Stallman, was the first software copyleft license to see extensive use, and continues to dominate in that area. Creative Commons, a non-profit organization founded by Lawrence Lessig, provides a similar license provision condition called ShareAlike. ==Reciprocity== Copyleft is a copyright licensing scheme in which an author surrenders some, but not all rights under copyright law. Instead of allowing a work to fall completely into the public domain (where no ownership of copyright is claimed), copyleft allows an author to impose some restrictions on those who want to engage in activities that would more usually be reserved by the copyright holder. Under copyleft, derived works may be produced provided they are released under the compatible copyleft scheme. The underlying principle is that one benefits freely from the work of others but any modifications one makes must be released under compatible terms. For this reason some copyleft licenses are also known as reciprocal licenses, they have also been described as "viral" due to their self-perpetuating terms. Under fair use, however, the copyleft license may be superseded, just like regular copyrights. Therefore, any person utilizing a copyleft-licensed source for their own work is free to choose any other license provided they meet the fair use standard.〔 〕 While copyright law gives software authors control over copying, distribution and modification of their works, the goal of copyleft is to give all users of the software the freedom to carry out these activities. In this way, copyleft licenses are distinct from other types of free software licenses, which do not guarantee that all subsequent recipients of the program receive these rights, or the source code needed to make them effective. In particular, permissive free software licenses such as BSD allow re-distributors to remove some or all these rights, and do not require the distribution of source code. Reuses of open source software which subvert the freedom of the software by adding extra restrictions are called open source hijacking. Other forms of open source licensing are susceptible to hijacking, so creators who desire that their work remain free may choose to use copyleft. The Open Directory Project (ODP, also known as DMOZ), which is created and maintained entirely by volunteer editors, can be considered an example of open-source hijacking. Since all of the top search engines use DMOZ, it is an extremely important public commodity which is entirely built by volunteer contributions. Despite this, the DMOZ has remained a commercial product for its entire existence. The economic incentives to work on copyleft content can vary. Traditional copyright law is designed to promote progress by providing economic benefits to creators. When choosing to copyleft their work, content creators may seek complementary benefits like recognition from their peers. The open-source culture had been described as a gift culture, where social status is determined by an individual's contributions. Working on free software may also be an outlet for programmers to fill a need they have noticed. For some creators, keeping their work open is an incentive in and of itself. For these programmers, preventing commercial enterprises from absorbing and selling their product is another incentive. Copyleft software has economic effects beyond the individual creators. The presence of quality copyleft software can force commercial producers to increase the quality of their products, which must compete with free software. This may also have the effect of preventing large commercial entities from applying monopoly prices. However, competition with proprietary software can also be a reason to forego copyleft. The Free Software Foundation recommends that when “widespread use of the code is vital for advancing the cause of free software,” allowing the code to be copied and used freely is more important than a copyleft. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Copyleft」の詳細全文を読む スポンサード リンク
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