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+<!-- This is the second edition of Free Software, Free Society: Selected Essays of Richard M. Stallman.
+
+Free Software Foundation
+
+51 Franklin Street, Fifth Floor
+
+Boston, MA 02110-1335
+Copyright C 2002, 2010 Free Software Foundation, Inc.
+Verbatim copying and distribution of this entire book are permitted
+worldwide, without royalty, in any medium, provided this notice is
+preserved. Permission is granted to copy and distribute translations
+of this book from the original English into another language provided
+the translation has been approved by the Free Software Foundation and
+the copyright notice and this permission notice are preserved on all
+copies.
+
+ISBN 978-0-9831592-0-9
+Cover design by Rob Myers.
+
+Cover photograph by Peter Hinely.
+ -->
+
+
+ <a name="What-Is-Copyleft_003f">
+ </a>
+ <h1 class="chapter">
+ 21. What Is Copyleft?
+ </h1>
+ <a name="index-GPL-2">
+ </a>
+ <a name="index-copyleft-_0028see-also-copyright_0029-3">
+ </a>
+ <a name="index-copylefted-software-_0028see-also-software_0029-1">
+ </a>
+ <p>
+ 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.
+ </p>
+ <a name="index-public-domain-software-_0028see-also-software_0029-4">
+ </a>
+ <p>
+ 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.
+ </p>
+ <p>
+ In the GNU Project, our aim is
+to give
+ <em>
+ all
+ </em>
+ 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.
+ </p>
+ <p>
+ Copyleft also provides an incentive for other programmers to add to
+free software. Important free programs such as the
+ <a name="index-GNU_002c-GNU-C_002b_002b-compiler">
+ </a>
+ GNU C++ compiler
+exist only because of this.
+ </p>
+ <p>
+ 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.
+ </p>
+ <p>
+ 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.
+ </p>
+ <p>
+ 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,
+ <em>
+ or any program derived from it,
+ </em>
+ but only if the
+distribution terms are unchanged. Thus, the code and the freedoms
+become legally inseparable.
+ </p>
+ <p>
+ 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.”
+ </p>
+ <p>
+ 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.”
+ </p>
+ <p>
+ 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. 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
+ <a href="http://gnu.org/licenses/gpl-faq.html">
+ http://gnu.org/licenses/gpl-faq.html
+ </a>
+ . You can also
+read about why the FSF gets copyright assignments from contributors,
+at
+ <a href="http://gnu.org/copyleft/why-assign.html">
+ http://gnu.org/copyleft/why-assign.html
+ </a>
+ .
+ </p>
+ <a name="index-libraries-_0028comp_002e_0029_002c-LGPL-and">
+ </a>
+ <a name="index-LGPL_002c-and-GNU-libraries">
+ </a>
+ <a name="index-GNU_002c-GNU-libraries">
+ </a>
+ <a name="index-libraries-_0028comp_002e_0029_002c-GNU-1">
+ </a>
+ <p>
+ An alternate form of copyleft, the GNU Lesser General Public License
+(LGPL), 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
+ <a href="http://gnu.org/philosophy/why-not-lgpl.html">
+ http://gnu.org/philosophy/why-not-lgpl.html
+ </a>
+ .
+ </p>
+ <a name="index-manuals_002c-FDL-and">
+ </a>
+ <a name="index-FDL-_0028see-also-both-manuals-and-documentation_0029-1">
+ </a>
+ <p>
+ The GNU Free Documentation License (FDL) 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.
+ </p>
+ <p>
+ The appropriate license is included in many manuals and in each GNU
+source code distribution.
+ </p>
+ <p>
+ 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.
+ </p>
+ <a name="index-LGPL_002c-altering-distribution-terms-to-GPL">
+ </a>
+ <p>
+ 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.
+ </p>
+ <p>
+ If you would like to copyleft your program with the GNU GPL or the GNU
+LGPL, please see the license instructions page, at
+ <a href="http://gnu.org/copyleft/gpl-howto.html">
+ http://gnu.org/copyleft/gpl-howto.html
+ </a>
+ , 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.
+ </p>
+ <p>
+ If you would like to copyleft your manual with the GNU FDL, please see
+the instructions at the end of the FDL text, and the GFDL
+instructions page, at
+ <a href="http://gnu.org/copyleft/fdl-howto.html">
+ http://gnu.org/copyleft/fdl-howto.html
+ </a>
+ . Again, partial
+copies are not permitted.
+ <a name="index-copylefted-software-_0028see-also-software_0029-2">
+ </a>
+ <a name="index-copyleft-_0028see-also-copyright_0029-4">
+ </a>
+ <a name="index-GPL-3">
+ </a>
+ </p>
+ <hr size="2"/>
+