Copyleft is a clause in copyright exploitation licenses, which stipulates that edits the work are only permitted if all changes are propagated only under the same or substantially the same license terms. You should prevent modified versions of the work are passed with restrictions on use, which has not the original. Copyleft requires that copies and adaptations are allowed in any way. However, for it makes itself seen No further statements about the nature and extent of the proper license and can therefore be used in very different content licenses.

Copyleft originally came on at any Free Software licenses. There enforces that further developments of a free primal program are again free and remain free. One speaks during copyleft so from a viral effect. It thus prevents Licensee run the program proprietary extensions in the proprietary domain. The most popular copyleft license is the GNU General Public License ( GPL). Later, the same principle was applied also for licenses for free content. Copyleft is not a necessary part of a free software license. For example, the BSD license has no copyleft, but including shared programs are free software.

Since the licensor is not bound himself to his own copyleft, he may publish new versions under a proprietary license or permit third parties to this.

Copyleft is a play on words that a contrast to the English phrase Copyright ( German: Copyright, literally " right to copy " ) by replacing " right" ( engl. right) by the counter word "left" ( engl. left ) was constructed at the same time left " let ( about ) " a grammatical form of being. Analogous to the interchange of the two terms is the ( copyright ) situation is reversed: By Copyleft the right to copy is granted in principle, as it is strictly prohibited by copyright. It's kind of a trick that uses the copyright in order to achieve the opposite of its usual purpose. In addition, the copyleft, the other freedoms of the license, eg in free software protects the freedom of access to the source code.


Edits a copyright legally the work of another, he shall receive according to case law a say on how the processing must be used. Was the original work nor for any freely copied, distributed, altered, etc., then transferred these freedoms not automatically editing. This attempt copyleft licenses to change. Since the original author has a say in the machining, it allows only the transfer of edits, if they are licensed to the same extensive rights to everyone. Copyleft is expected to prevent that free works to the starting material proprietary content.

The copyleft method was first used by the Free Software Foundation for the GNU licenses (GPL, LGPL and GFDL ). Meanwhile, the process, however, also used by other organizations. This concerns, for example, various music licenses such as the OAL of the Electronic Frontier Foundation, as well as attempts to create a workable for all business types copyleft license, such as the Design Science License ( DSL) or the " Share Alike " license of the project Creative Commons (CC- BY- SA and CC -BY -NC -SA, the latter of which does not grant all the subsumed under free software freedoms because of the restriction to non-commercial use).

Strength of a copyleft

Depending on the degree, are affected as derivative works of the license with the works, which included another work, a distinction is made between strong and weak copyleft - the transition is smooth. Thus, in a strong copyleft are of a piece of music that is used for a film that demands that even the film itself is called by this license, while a weak copyleft would not require this, but only for changes to the play itself, about the lyrics.

Historically, this term first arose in the GPL and LGPL, which are mainly used for software. In particular, do libraries that have been released under a license with a strong copyleft (GPL ), linked only in programs that also include, among other under the GPL. The question of how far-reaching a copyleft affects whether and how far it also affects the function calls to dynamic libraries or runtime loaded modules added, is partly interpretation of the license and so sometimes controversially discussed. In particular, under proprietary licenses related device drivers that are loaded as a module added in areas placed under the GPL Linux kernel, frequently lead to differences of opinion of the respective manufacturers with the kernel developers.

The Mozilla Public License is considered very weak copyleft, the LGPL and the GPL as a weak regarded as a strong copyleft (for example, from the FSF ).


Open Group and developer of various BSD - lines seen by the prohibition to publish modified versions under a more restrictive license that restricted freedom of inappropriate and advised instead to free licenses without copyleft (such as BSD license or MIT license ).

By copyleft licenses can quickly incompatibilities, even to free non- copyleft licenses, occur. For example, was the original four- clause BSD license, which was later modified according to Dreiklausigen, incompatible with the GPL. Other free licenses for programs are almost exclusively dual licensed with the GPL, that is, the licensee has the choice between the two licenses. Examples are the Artistic License (non- copyleft ) and Others in Perl and the Mozilla Public License (weak copyleft ) and Others at Mozilla. Alternatively, the licenses can allow himself that the work may be put under another license (eg GPL with the LGPL version 2.1) or define themselves as exceptions to an existing license (eg LGPL Version 3 Draft 1 as an exception GPL 3).