Copyleft is a type of license that attempts to ensure that the public retains the freedom to use, modify, extend and redistribute a creative work and all derivative works (i.e., works based on or derived from it) rather than to restrict such freedoms.
This is accomplished by the copyright holder granting irrevocable permission to the public to copy and redistribute the work in the same or modified form, but with the conditions that all such redistributions (1) make the work available in a form that facilitates further modification and (2) use the same license.
A copyright is a designation by a government that grants the author of a creative work (e.g., a musical composition, painting, poem, product design, movie or computer software) the exclusive (but transferable) right to copy or perform that work. Its original purpose was to provide a financial incentive for producing such works in order to benefit society as a whole. Copyright does not protect facts, discoveries, ideas, systems or methods of operation, although it can protect the way they are expressed.
The term copyleft is a play on the word copyright, and it may superficially appear that it is because the concept is favored by some people who consider themselves to be leftists in a political sense. However, in reality, people from throughout the political spectrum, including many who consider themselves to be true conservatives1, have high regard for this concept because it is not at all about socialism or other political philosophies; rather, it is about freedom, the advance of computer technology, and benefiting the economy and society as a whole.
In the case of computer software, the form that facilitates further modification is source code, and thus copyleft licenses require that the source code be made freely available to anyone who wants it. Source code (also referred to as source or code) is the version of software (usually an application program or an operating system) as it is originally written (i.e., typed into a computer) by a human in plain text (i.e., human readable alphanumeric characters). There is no requirement to provide the executable (i.e., ready-to-run) version, as it is a relatively easy matter to compile (i.e., convert) source code into runnable programs.
Most copylefted software is issued under the GNU General Public License (GPL), which was the first copyleft license and by far the most commonly used license for free software. Free software is software whose source code is freely available to anyone to use for any purpose, including studying, copying, modifying, extending and giving away.
Not all free software licenses are copyleft licenses. For example, BSD style licenses and the MIT license are not, because they do not require that redistributions of modified versions in compiled form make the source code available along with them.
Copyleft is a somewhat controversial issue. Those objecting to it claim that it is viral in nature (because any works derived from a such works must themselves be copylefted) and that it contaminates all derivative works by forcing them to likewise be subject to copyleft. They claim that this destroys intellectual property. This term viral is considered derogatory, because it compares copylefted works to harmful computer viruses and biological viruses. The most vociferous opponent of copyleft has been Microsoft Corporation, which, according to advocates of copyleft, is because it feels threatened by Linux and other free software.
In addition to becoming popular because of its ideology, copyleft has also proved to be an extremely practical concept for the promotion of the development of high quality computer software. This is because it assures software developers that no dominant company will be able to take over their work and that such work will always remain available to everyone to use. The success is evidenced by not only the vast number of copyleft projects currently in various stages of development, but also by the substantial success of individual projects, such as Linux, MySQL, Open Office, Ruby and Blender.
The origin of the term copyleft is not certain. It may have first appeared in a message contained in Tiny BASIC, a free version of the Basic programming language that was written by Dr. Li Chen Wang in the late 1970s.
Created June 24, 2006.