yields software freedom for its users.

Carefully note that software truly in the public domain is \emph{not} licensed

in any way.  It is confusing to say software is ``licensed for the

public domain,'' or any phrase that implies the copyright holder gave

express permission to take actions governed by copyright law.

Copyright holders who state that they are releasing their code into

the public domain are effectively renouncing copyright controls on

the work.  The law gave the copyright holders exclusive controls over the

work, and they chose to waive those controls.  Software that is, in

this sense, in the public domain

is conceptualized by the developer as having no copyright and thus no license. The software freedoms discussed in

Section~\ref{Free Software Definition} are all granted because there is no

legal system in play to take them away.

Admittedly, a discussion of public domain software is an oversimplified

example.  

Because copyright controls are usually automatically granted and because, in

some jurisdictions, some copyright controls cannot be waived (see

Section~\ref{non-usa-copyright} for further discussion), many copyright

holders sometimes incorrectly believe a work has been placed in the public

domain.  Second, due to aggressive lobbying by the entertainment industry,

the ``exclusive Right'' of copyright, that was supposed to only exist for

``Limited Times'' according to the USA Constitution, appears to be infinite:

simply purchased on the installment plan rather than in whole.  Thus, we must

assume no works of software will fall into the public domain merely due to

the passage of time.

Nevertheless, under USA law it is likely that the typical

disclaimers of copyright or public domain dedications we see in the

Free Software world would be interpreted by courts as copyright

abandonment, leading to a situation in which the user effectively receives a

maximum grant of copyright freedoms, similar to a maximally-permissive

Free Software license.

The best example of software known to truly be in the public domain is software

that is published by the USA government.  Under

\href{http://www.law.cornell.edu/uscode/text/17/105}{17 USC 101 \S~105}, all

works published by the USA Government are not copyrightable in the USA.

\subsection{Why Copyright Free Software?}

If simply disclaiming copyright on software yields Free Software, then it

stands to reason that putting software into the public domain is the

easiest and most straightforward way to produce Free Software. Indeed,

some major Free Software projects have chosen this method for making their

software Free. However, most of the Free Software in existence \emph{is}

Software released into the public domain does grant freedom to those users

who receive the standard versions on which the original author disclaimed

copyright. However, since the work is not copyrighted, any nontrivial

modification made to the work is copyrightable.

Free Software released into the public domain initially is Free, and

perhaps some who modify the software choose to place their work into the

public domain as well. However, over time, some entities will choose to

proprietarize their modified versions. The public domain body of software

feeds the proprietary software. The public commons disappears, because

fewer and fewer entities have an incentive to contribute back to the

commons. They know that any of their competitors can proprietarize their

enhancements. Over time, almost no interesting work is left in the public

domain, because nearly all new work is done by proprietarization.

A legal mechanism is needed to redress this problem. FSF was in fact

originally created primarily as a legal entity to defend software freedom,

and that work of defending software freedom is a substantial part of

its work today. Specifically because of this ``embrace, proprietarize and

extend'' cycle, FSF made a conscious choice to copyright its Free Software,

and then license it under ``copyleft'' terms. Many, including the

developers of the kernel named Linux, have chosen to follow this paradigm.

\label{copyleft-definition}

Copyleft is a strategy of utilizing copyright law to pursue the policy goal

of fostering and encouraging the equal and inalienable right to copy, share,

modify and improve creative works of authorship.  Copyleft (as a general

term) describes any method that utilizes the copyright system to achieve the

aforementioned goal.  Copyleft as a concept is usually implemented in the

details of a specific copyright license, such as the

\hyperref[GPLv3-full-text]{GNU General Public License (GPL)} and the Creative

Commons Attribution Share Alike License (the latter of which is the license

of this work itself).  Copyright holders of creative work can unilaterally

implement these licenses for their own works to build communities that

collaboratively share and improve those copylefted creative works.

Copyleft uses functional parts of the copyright system to achieve an unusual

result (legal protection for free sharing). Copyleft modifies, or ``hacks''

copyright law, which is usually employed to strengthen the rights of authors

or publishers, to strengthen instead the rights of users.  Thus, Copyleft is

a legal strategy and mechanism to defend, uphold and propagate software

freedom. The basic technique of copyleft is as follows: copyright the

software, license it under terms that give all the software freedoms, but use

the copyright law controls to ensure that all who receive a copy of the

software have equal rights and freedom. In essence, copyleft grants freedom,

but forbids others to forbid that freedom to anyone else along the

distribution and modification chains.

Copyleft's ``reciprocity'' or ``share and share alike'' rule protects both

developers, who avoid facing a ``prioritized'' competitor of their project,

and users, who can be sure that they will have all four software freedoms ---

not only in the present version of the program they use, but in all its