not restrict public performance.} By law (in the USA and in most other

jurisdictions), the copyright holder (most typically, the author) of the work controls

how others may copy, modify and/or distribute the work. For proprietary

software, these controls are used to prohibit these activities. In addition,

proprietary software distributors further impede modification in a practical

sense by distributing only binary code and keeping the source code of the

software secret.

Copyright is not a natural state, it is a legal construction. In the USA, the

Constitution permits, but does not require, the creation of copyright law as

federal legislation.  Software, since it is an ``original work of authorship

fixed in any tangible medium of expression ...  from which they can be

perceived, reproduced, or otherwise communicated, either directly or with the

aid of a machine or device'' (as stated in

\href{http://www.law.cornell.edu/uscode/text/17/102}{17 USC \S~102}), is thus

covered by the statute, and is copyrighted by default.

However, software, in its natural state without copyright, is Free

Software. In an imaginary world with no copyright, the rules would be

different. In this world, when you received a copy of a program's source

code, there would be no default legal system to restrict you from sharing it

with others, making modifications, or redistributing those modified

versions.\footnote{Note that this is again an oversimplification; the

  complexities with this argument are discussed in

  Section~\ref{software-and-non-copyright}.}

Software in the real world is copyrighted by default and is automatically

covered by that legal system.  However, it is possible to move software out

of the domain of the copyright system.  A copyright holder can often

\defn{disclaim} their copyright. (For example, under USA copyright law

it is possible for a copyright holder to engage in conduct resulting

in abandonment of copyright.)  If copyright is disclaimed, the software is

effectively no longer restricted by copyright law.   Software not restricted by copyright is in the

``public domain.''

\subsection{Public Domain Software}

In the USA and other countries that

are parties to the Berne Convention on Copyright, software is copyrighted

automatically by the author when she fixes the software in a tangible

medium.  In the software world, this usually means typing the source code

of the software into a file.

Imagine if authors could truly disclaim those default controls of copyright

law.  If so, the software is in the public domain --- no longer covered by

copyright.  Since copyright law is the construction allowing for most

restrictions on software (i.e., prohibition of copying, modification, and

redistribution), removing the software from the copyright system usually

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

future improved versions.

Copyleft is a general concept. Much like ideas for what a computer might

do must be \emph{implemented} by a program that actually does the job, so

too must copyleft be implemented in some concrete legal structure.

``Share and share alike'' is a phrase that is used often enough to explain the

concept behind copyleft, but to actually make it work in the real world, a

true implementation in legal text must exist, written as a ``copyright

license''.  The GPL implements the concept of copyleft for software-oriented

and other functional works of a technical nature.  The ``CC BY SA'' license

implements copyleft for works of textual, musical and visual authorship, such

as this tutorial.

Copyleft advocates often distinguish between the concept of a ``strong

copyleft'' or a ``weak copyleft''.  However, ``strong vs. weak'' copyleft is

not a dichotomy, it's a spectrum.  The strongest copylefts strive to the

exclusive rights that copyright  grants to authors as extensively as possible

to maximize software freedom.  As a copyleft gets ``weaker'', the copyleft

license typically makes ``trade offs'' that might impede software freedom,

but reach other tactic goals for the community of users and developers of the

work.

In other words, strong copyleft licenses place the more requirements on how

``the work'' is licensed.  The unit of copyright law is ``the work''.  In

that sense, the ``work'' referenced by the licenses is anything that can be

copyrighted or will be subject to the terms of copyright law.  Strong

copyleft licenses exercise their scope fully.  Anything which is ``a work''

or a ``work based on a work'' licensed under a strong copyleft is subject to

its requirements, including the requirement of complete, corresponding source

code\footnote{Copyleft communities' use of the term ``strong copyleft'' is

  undoubtedly imprecise.  For example, most will call the GNU GPL a ``strong

  copyleft'' license, even though the GPL itself has various exceptions, such

  as the \hyperref[GPLv3-system-library-exception]{GPLv3's system library

    exception} written into the text of the license itself.  Furthermore, the

  copyleft community continues to debate where the a license cross the line

  from ``strong copyleft'' to ``license that fails to respect software

  freedom'', although ultimately these debates are actually regarding whether

  the license fits \hyperref[Free Software Definition]{Free Software

    definition} at all.}.  Thus, copyleft licenses, particularly strong ones,

seek to ensure the same license covers every version of ``work based on the

work'', as recognized by local copyright law, and thereby achieve the

specific strategic policy aim of ensuring software freedom for all users,

developers, authors, and readers who encounter the copylefted work.

\subsection{Software and Non-Copyright Legal Regimes}

\label{software-and-non-copyright}

The use, modification and distribution of software, like many endeavors,