Copyleft is ==a licensing practice that uses copyright law to grant users the right to freely modify, share, and redistribute creative works or software, provided that all derivative works are released under the same license terms==. Coined by Richard Stallman in the 1980s, it reverses traditional copyright to ensure software remains open, with the GNU General Public License (GPL) being a primary example.
Common Copyleft Licenses
Difference from Permissive Licensing
While copyleft licenses (like GPL) compel you to share improvements, “permissive” licenses (like MIT or Apache) allow creators to modify the code and distribute it without sharing the changes. [1, 2, 3, 4, 5]
see https://en.wikipedia.org/wiki/Copyleft
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 work the freedom to carry out all of these activities. The Free Software Definition lists “four essential freedoms” of software use:[7][14]
Similar terms are present in the Open Source Definition, a separate definition that contains similar freedoms. The vast majority of copyleft licenses satisfy both definitions, that of the Free Software Definition and Open Source Definition.[7] By guaranteeing viewers and users of a work the freedom and permission to reproduce, adapt, or distribute it, copyleft licenses are distinct from other types of copyright licenses that limit such freedoms.
Instead of allowing a work to fall completely into the public domain, where no ownership of copyright is claimed, copyleft allows authors to impose restrictions on the use of their work. One of the main restrictions imposed by copyleft is that derived works must also be released under a compatible copyleft license.[7]
This is due to the underlying principle of copyleft: that anyone can benefit freely from the previous work of others, but that any modifications to that work should benefit everyone else as well, and thus must be released under similar terms. For this reason, copyleft licenses are also known as reciprocal licenses: any modifiers of a copyleft-licensed work are expected to reciprocate the author’s action of copyleft-licensing the software by also copyleft-licensing any derivatives they might have made. Because of this requirement, copyleft licenses have also been described as “viral” due to their self-perpetuating terms.[15]
In addition to restrictions on copying, copyleft licenses address other possible impediments. They ensure that rights cannot be later revoked, and require the work and its derivatives to be provided in a form that allows further modifications to be made. In software, this means requiring that the source code of the derived work be made available together with the software itself.[7]
The economic incentives to work on copyleft works can vary. Copyright law was originally intended to promote progress by encouraging economic benefits to authors. When choosing to copyleft their work, authors may seek complementary benefits like recognition from their peers.
In the world of computer programming, copyleft-licensed computer programs are often created by programmers to fill a need they have noticed. Such programs are often published with a copyleft license simply to ensure that subsequent users can also freely use modified versions of that program. This is especially true for authors who wish to prevent proprietarization, or the act of reusing free software and then adding extra restrictions to it, an action prevented by copyleft-licensing the software. Some businesses, such as Elastic[16], feel that preventing commercial enterprises from using and then selling their product under a proprietary license is also an incentive.
Furthermore, the open-source culture of programming has been described as a gift economy, where social power is determined by an individual’s contributions.[17] Contributing to or creating open-source, copyleft-licensed software of high quality can lead to contributors gaining valuable experience and can lead to future career opportunities.[18]
Copyleft software has economic effects beyond individual authors. The presence of quality copyleft software can force proprietary software developers to increase the quality of their software to compete with free software.[19] This may also have the effect of preventing monopolies in areas dominated by proprietary software. However, competition with proprietary software can also be a reason to forgo copyleft. The Free Software Foundation recommends that in special situations, such as libraries that implement free data formats, that are competing against entrenched proprietary formats, “if you are aiming to convince proprietary application developers to use the library for the free format, you would need to make that easy by licensing the library under a weak license” [20].