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}

copyrighted. In most cases (particularly in those of FSF and the GNU

Project), this was done due to very careful planning.

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 fully 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 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 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. The GPL is the primary

implementation of copyleft in copyright licensing language.

\subsection{Software and Non-Copyright Legal Regimes}

\label{software-and-non-copyright}

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

simultaneously interacts with multiple different legal regimes.  As was noted

early via footnotes, copyright is merely the \textit{most common way} to

restrict users' rights to copy, share, modify and/or redistribute software.

However, proprietary software licenses typically use every mechanism

available to subjugate users.  For example:

\begin{itemize}

\item Unfortunately, despite much effort by many in the software freedom

  community to end patents that read on software (i.e., patents on

  computational ideas), they still ultimately exist.  As such, a software

  program might otherwise be seemly unrestricted, but a patent might read on

  the software and ruin everything for its users.\footnote{See

  \S\S~\ref{gpl-implied-patent-grant},~\ref{GPLv2s7},~\ref{GPLv3s11} for more

  discussion on how the patent system interacts with copyleft, and read

  Richard M.~Stallman's essay,

  \href{http://www.wired.com/opinion/2012/11/richard-stallman-software-patents/}{\textit{Let’s

      Limit the Effect of Software Patents, Since We Can’t Eliminate Them}}

  for more information on the problems these patents present to society.}

\item Digital Restrictions Management (usually called \defn{DRM}) is often

  used to impose technological restrictions on users' ability to exercise

  software freedom that they might otherwise be granted\footnote{See

    \S~\ref{GPLv3-drm} for more information on how GPL deals with this issue.}.

  The simplest (and perhaps oldest) form of DRM, of course, is separating

  software source code (read by humans), from their compiled binaries (read

  only by computers).  Furthermore,

  prohibits users legally from circumventing some of these DRM systems.

\item Most EULAs also include a contractual agreement that bind users further

  by forcing them to agree to a contractual, prohibitive software license

  before ever even using the software.

\end{itemize}

Thus, most proprietary software restricts users via multiple interlocking

legal and technological means.  Any license that truly respect the software

freedom of all users must not only grant appropriate copyright permissions,

but also \textit{prevent} restrictions from other legal and technological

means like those listed above.

\subsection{Non-USA Copyright Regimes}

\label{non-usa-copyright}

Generally speaking, copyright law operates similarly enough in countries that

have signed the Berne Convention on Copyright, and software freedom licenses

have generally taken advantage of this international standardization of

copyright law.  However, copyright law does differ from country to country,

and commonly, software freedom licenses like GPL must be considered under the

copyright law in the jurisdiction where any licensing dispute occurs.

Those who are most familiar with the USA's system of copyright often are

surprised to learn that there are certain copyright controls that cannot be

waived nor disclaimed.  Specifically, many copyright regimes outside the USA

recognize a concept of moral rights of authors.  Typically, moral rights are

fully compatible with respecting software freedom, as they are usually

centered around controls that software freedom licenses generally respect,

such as the right of an authors to require proper attribution for their work.

\section{A Community of Equality}

The previous section described the principles of software freedom, a brief

introduction to mechanisms that typically block these freedoms, and the

simplest ways that copyright holders might grant those freedoms to their

users for their copyrighted works of software.  The previous section also

introduced the idea of \textit{copyleft}: a licensing mechanism to use

copyright to not only grant software freedom to users, but also to uphold

those rights against those who might seek to curtail them.

Copyleft, as defined in \S~\ref{copyleft-definition}, is a general term this

mechanism.  The remainder of this text will discuss details of various

real-world implementations of copyleft -- most notably, the GPL\@.

This discussion begins first with some general explanation of what the GPL is

able to do in software development communities.  After that brief discussion

in this section, deeper discussion of how GPL accomplishes this in practice

follows in the next chapter.

Simply put, though, the GPL ultimately creates an community of equality for

both business and noncommercial users.

\subsection{The Noncommercial Community}

A GPL'd code base becomes a center of a vibrant development and user

community.  Traditionally, volunteers, operating noncommercially out of

keen interest or ``scratch an itch'' motivations, produce initial versions

of a GPL'd system.  Because of the efficient distribution channels of the

Internet, any useful GPL'd system is adopted quickly by noncommercial

users.

Fundamentally, the early release and quick distribution of the software

gives birth to a thriving noncommercial community.  Users and developers

begin sharing bug reports and bug fixes across a shared intellectual

commons.  Users can trust the developers, because they know that if the

developers fail to address their needs or abandon the project, the GPL

ensures that someone else has the right to pick up development.

Developers know that the users cannot redistribute their software without

passing along the rights granted by GPL, so they are assured that every

one of their users is treated equally.

Because of the symmetry and fairness inherent in GPL'd distribution,

nearly every GPL'd package in existence has a vibrant noncommercial user

and developer base.

\subsection{The Commercial Community}

By the same token, nearly all established GPL'd software systems have a

vibrant commercial community.  Nearly every GPL'd system that has gained

wide adoption from noncommercial users and developers eventually begins

to fuel a commercial system around that software.

For example, consider the Samba file server system that allows Unix-like

systems (including GNU/Linux) to serve files to Microsoft Windows systems.

Two graduate students originally developed Samba in their spare time and

it was deployed noncommercially in academic environments\footnote{See

  \href{http://turtle.ee.ncku.edu.tw/docs/samba/history}{Andrew Tridgell's

    ``A bit of history and a bit of fun''}}.  However, very

soon for-profit companies discovered that the software could work for them

as well, and their system administrators began to use it in place of

Microsoft Windows NT file-servers.  This served to lower the cost of

running such servers by orders of magnitude. There was suddenly room in

Windows file-server budgets to hire contractors to improve Samba.  Some of

the first people hired to do such work were those same two graduate