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 control of copyright

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

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

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