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1. What Is Copyleft? {#what-is-copyleft .chapter}
====================
@firstcopyingnotice{{Copyright © 1996–2009, 2013 Free Software
Foundation, Inc.\
{This essay was originally published on , in 1996. This
version is part of @fsfsthreecite} 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
(@pageref{GPL}). 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 . You can also read
about why the FSF gets copyright assignments from contributors, at
.
An alternate form of copyleft, the GNU Affero General Public License
(AGPL), is designed for programs that are likely to be used on servers.
It ensures that modified versions used to implement services available
to the public are released as source code to the public.
An alternate form of copyleft, the GNU Lesser General Public License
(LGPL) (@pageref{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 .
The GNU Free Documentation License (FDL) (@pageref{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.
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
, 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 (@pageref{FDL
Instructions}), and the GFDL instructions page, at
. Again, partial copies are not
permitted.
It is a legal mistake to use a backwards C in a circle instead of a
copyright symbol. Copyleft is based legally on copyright, so the work
should have a copyright notice. A copyright notice requires either the
copyright symbol (a C in a circle) or the word “Copyright.”
A backwards C in a circle has no special legal significance, so it
doesn’t make a copyright notice. It may be amusing in book covers,
posters, and such, but be careful how you represent it in a web page!
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