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

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

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.

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.

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This principle often causes theoretical or

speculative dispute among lawyers, because ``the work'' is not a unit of

computer programming. In order to determine whether a ``routine'' an

``object'', a ``function'', a ``library'' or any other unit of software is

part of one ``work'' when combined with other GPL’d code, we must ask a

question that copyright law will not directly answer in the same technical

terms.  (This issue

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\label{GPLv3-system-library-exception}