21. What Is Copyleft?

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Copyleft is a general method for making a program (or +other work) free, and requiring all modified and extended versions of the +program to be free as well. +

+ +

The simplest way to make a program free software is to put it in the +public domain, uncopyrighted. This allows people to +share the program and their improvements, if they are so minded. But +it also allows uncooperative people to convert the program into +proprietary software. They can make changes, many or few, +and distribute the result as a proprietary product. People who +receive the program in that modified form do not have the freedom that +the original author gave them; the middleman has stripped it away. +

In the GNU Project, our aim is +to give all users the freedom to redistribute and change GNU +software. If middlemen could strip off the freedom, we might have +many users, but those users would not have freedom. So instead of +putting GNU software in the public domain, we “copyleft” +it. Copyleft says that anyone who redistributes the software, with or +without changes, must pass along the freedom to further copy and +change it. Copyleft guarantees that every user has freedom. +

Copyleft also provides an incentive for other programmers to add to +free software. Important free programs such as the + +GNU C++ compiler +exist only because of this. +

Copyleft also helps programmers who want to contribute improvements to +free software get permission to +do so. These programmers often work for companies or universities +that would do almost anything to get more money. A programmer may +want to contribute her changes to the community, but her employer may +want to turn the changes into a proprietary software product. +

When we explain to the employer that it is illegal to distribute the +improved version except as free software, the employer usually decides +to release it as free software rather than throw it away. +

To copyleft a program, we first state that it is copyrighted; then we +add distribution terms, which are a legal instrument that gives +everyone the rights to use, modify, and redistribute the program’s +code, or any program derived from it, but only if the +distribution terms are unchanged. Thus, the code and the freedoms +become legally inseparable. +

Proprietary software developers use copyright to take away the users’ +freedom; we use copyright to guarantee their freedom. That’s why we +reverse the name, changing “copyright” into +“copyleft.” +

Copyleft is a way of using of the copyright on the program. It +doesn’t mean abandoning the copyright; in fact, doing so would make +copyleft impossible. The “left” in +“copyleft” is not a reference to the verb “to +leave”—only to the direction which is the inverse of +“right.” +

Copyleft is a general concept, and you can’t use a general concept +directly; you can only use a specific implementation of the concept. +In the GNU Project, the specific distribution terms that we use for +most software are contained in the GNU General Public License (p. @refx{GPL-pg}{). The GNU General Public License is often called the GNU GPL for +short. There is also a Frequently Asked Questions page about the GNU +GPL, at http://gnu.org/licenses/gpl-faq.html. You can also +read about why the FSF gets copyright assignments from contributors, +at http://gnu.org/copyleft/why-assign.html. +

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An alternate form of copyleft, the GNU Lesser General Public License +(LGPL) (p. @refx{LGPL-pg}{), applies to a few (but not all) GNU libraries. To +learn more about properly using the LGPL, please read the article +“Why You Shouldn’t Use the Lesser GPL for Your Next Library,” +available at http://gnu.org/philosophy/why-not-lgpl.html. +

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The GNU Free Documentation License (FDL) (p. @refx{FDL-pg}{) is a form of +copyleft intended for use on a manual, textbook or other document to +assure everyone the effective freedom to copy and redistribute it, +with or without modifications, either commercially or noncommercially. +

The appropriate license is included in many manuals and in each GNU +source code distribution. +

All these licenses are designed so that you can easily apply them to +your own works, assuming you are the copyright holder. You don’t have +to modify the license to do this, just include a copy of the license +in the work, and add notices in the source files that refer properly +to the license. +

+ +

Using the same distribution terms for many different programs makes it +easy to copy code between various different programs. When they all +have the same distribution terms, there is no problem. The Lesser +GPL, version 2, includes a provision that lets you alter the +distribution terms to the ordinary GPL, so that you can copy code into +another program covered by the GPL. Version 3 of the Lesser GPL is +built as an exception added to GPL version 3, making the compatibility +automatic. +

If you would like to copyleft your program with the GNU GPL or the GNU +LGPL, please see the license instructions page, at +http://gnu.org/copyleft/gpl-howto.html, for advice. +Please note that you must use the entire text of the license you +choose. Each is an integral whole, and partial copies are not +permitted. +

If you would like to copyleft your manual with the GNU FDL, please see +the instructions at the end of the FDL text (p. @refx{FDL Instructions-pg}{), and the GFDL +instructions page, at +http://gnu.org/copyleft/fdl-howto.html. Again, partial +copies are not permitted. + + + +

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