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The simplest way to make a program free is to put it in the public domain, uncopyrighted. But this permits proprietary modifications, denying others the freedom to use and redistribute improvements; it is contrary to the intent of increasing the total amount of free software. To prevent this, copyleft uses copyrights in a novel manner. 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 rights become legally inseparable.
The copyleft used by the GNU Project is made from the combination of 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. An alternate form, the GNU Library General Public License (LGPL), applies to a few GNU libraries. This license permits linking the libraries into proprietary executables under certain conditions. The appropriate license is included in all GNU source code distributions and many manuals. Printed copies are available upon request.
We strongly encourage you to copyleft your programs and documentation, and we have made it as simple as possible for you to do so. The details on how to apply either license appear at the end of each license.
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