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/kop'ee-left/ (A play on "copyright") The copyright notice and General Public License applying to the works of the Free Software Foundation, granting reuse and reproduction rights to everyone.

Typically copyrights take away freedoms; copyleft preserves them. It is a legal instrument that requires those who pass on a program to include the rights to use, modify, and redistribute the code; the code and the freedoms become legally inseparable.

The copyleft used by the GNU Project combines a regular copyright notice and the "GNU General Public License" (GPL). The GPL is a copying license which basically says that you have the aforementioned freedoms. The license is included in each GNU source code distribution and manual.

See also General Public Virus.


A requirement in the GNU GPL software license and other "free" software licenses that anyone who redistributes the software does so under the same license and also includes the source code. The "free" means free of restrictions (see free software). The copyleft clauses were written to support copyright laws, not eliminate them.

Strong Vs. Weak
A "strong copyleft" license, such as the GNU GPL license, applies to all derived works and software components in the package. A "weak copyleft" license, such as the GNU Lesser GPL, applies only to the original copylefted work.

Full Vs. Partial
"Full copyleft" means that all of the work may be modified, whereas "partial copyleft" restricts some parts of the work from being altered. See GNU General Public License and copyright.
References in periodicals archive ?
FOSS's copyleft licensing structure offered novel terms with the promise of greater social benefit than proprietary software.
One of the effects of copyleft licensing is that it may generate long chains of distributions in which software projects may fork into others, with the originating source code becoming entangled in licensing webs.
GNU/Linux is based on a copyleft licensing system, which specifies not only that all of the GNU/Linux code is open -- that is, completely viewable to, and copyable by, anyone -- but that anyone who makes an addition or otherwise modifies the underlying code must make their code available according to the same copyleft license.
From public domain in copyright, the open access or copyleft licensing to the multiple and complex authorship resulting from online wiki creation and new forms of cohousing based on common spaces and property, all rely on the lack or limitation of exclusive rights and the accommodation of symmetric entitlements of other individuals.
There are other important characteristics of a chosen license to consider as well, for example, whether or not a patent commons surrounding the project is desirable, (115) or whether it is a priority to preserve all derivative works of the project as free software using the legal force of copyleft licensing.