Copyleft

Copyleft refers to a concept invented by Richard Stallman of the Free Software Foundation in 1984: the application of copyright laws to ensure uncompromised public freedom to manipulate, improve, and redistribute specific information and all its derivative works. Initially only designed for software distribution, the concept is now also being used for other types of material.

In copyleft, the copyright holder grants an irrevocable license to the recipient of a copy, permitting the redistribution (including sale) of possibly modified further copies, under the condition that all those copies carry the same license and are made available in a form which facilitates modification.

Some other free software licenses as well as so-called open source licenses, such as those used by BSD and the X Window System, are not copyleft licenses because they do not extend to derivative works and include no requirement to make source code available. It is a matter of debate whether these licenses provide a larger or smaller degree of freedom than copyleft licenses. While the former licenses attempt to maximize the freedom of the initial recipient, the latter licenses try to maximize the freedom of all potential recipients in the future.

Copyleft licenses are sometimes referred to as viral copyright licenses, because any works derived from a copylefted work must themselves be copylefted. This term is considered derogatory, as it compares copylefted works to harmful computer viruses. However, the analogy between copyleft and computer viruses is not close; as advocates of copyleft point out, computer viruses infects computers without the awareness of the user, whereas the copyleft actually grants the user certain permissions to distribute modified programs, which is otherwise not allowed under copyright law, and under most proprietary licenses.

The concept of copyleft arose when Stallman was working on a LISP interpreter. Symbolics asked to use the LISP interpreter, and Stallman agreed to supply them with a public domain version of his work. Symbolics extended and improved the LISP interpreter, but when Stallman wanted access to the improvements that Symbolics had made to his interpreter, Symbolics refused. Stallman then proceeded to create a software license that would prevent this behavior.

The term "Copyleft" comes from the phrase "Copyleft--all rights reversed", which Don Hopkins wrote in a message to Stallman in 1984 and which is intended as a double pun on the phrase "Copyright--all rights reserved".

Free software licenses which are examples of copyleft licenses include the GNU General Public License, the GNU Lesser General Public License, the Mozilla Public License, and the Q Public License. Copyleft licenses for materials other than software include the Open Content License, and the GNU Free Documentation License. The latter is being used for the contents of this encyclopedia.