Copyleft is a group of licenses that can be applied to any kind of creation (art, content, software…). In a non-legal sense, the copyleft is the opposite of copyright: it wants to ensure that any person who receives a copy of a work can use, modify and redistribute this work and its derived versions.

Even though copyleft is a free software license, some of these licenses are different from copyleft, like the Public Domain license. That is because while the Public Domain license allows the software creator to do almost anything with the licensed object (except removing its original copyright), copyleft does not allow him to re-distribute the versions with another license: all copyleft derivate material must have a copyleft license. The term copyleft comes from the open source communities, as wordplay from copyright, but the concept was introduced by Richard Stallman. Stallman defined the copyleft in function of the four freedoms that the user can exercise:

  • Freedom 0: there are no restrictions when using the program.
  • Freedom 1: the user is able to study and adapt it to the particular needs.
  • Freedom 2: the user is able to redistribute it.
  • Freedom 3: the user is able to improve it and to publish the improvements.

Some copyleft examples are the GNU General Public License for software and some of the Creative Commons for works and contents. In the following paragraphs a deeper approximation to copyleft will be exposed.


We can find the origins of copyleft at the 1970s. The computer programmers started using this term to note the diffusion freedom of certain software, mostly between students.

Some years later, at the 80s, the MIT professor Richard Stallman noticed how some companies were taking advantage of this public domain software, acquiring it for free, improving it and refusing the authors of the original version to view the improvements. In 1981 Stallman started thinking about how to eradicate this behavior, which he called software hoarding.


A few years later, Stallman ceased teaching in order to fully devote himself to the GNU (Gnu’s not UNIX) project. This project included the General Public License (or GPL), the main goal of which is to prevent the GPL material to be legally subject to copyright.

From then on, there have been some improvements on GNU license in order to release the material from the copyright and make it more accessible for anybody. Also, some other similar licenses have appeared, like the Creative Commons Share Alike.


The term copyleft was born as a humoristic definition of copyright, playing with the meaning of right (in his typical use and in the political use). The copyleft would be the freedom claim in front of the author rights. Moreover, this left is also used as the participle of to leave. All the creations spread within this philosophy are left to the users’ disposal.

There are two versions about the origin of this term:

According to some sources, the word comes from the software Tiny BASIC, written and freely distributed by Dr. Li-Chen Wang in 1976. This program included the sentence: “@Copyleft. All Wrongs Reserved” in “Copyright. All Rights Reserved”.

On the other hand, Richard Stallman assures that this term was created by Don Hopkins, who sent him a letter in 1984 or 1985 with the sentence “Copyleft. All rights reversed”. These same words were written in the early 1970s in the Principia Discordia, text that may have inspired Hopkins.

Weak and strong copyleft

We can divide copyleft license between two types, depending on how restrictive they are. The least limiting one is called weak copyleft, while the other one, the strong copyleft, is the most restrictive one.

  • We call weak copyleft those free software licenses that allow the underlying objects to be redistributed with other licenses, depending on its derivation way. Weak copyleft licensed software can be linked from a different licensed program; otherwise, it can be incorporated into larger software. The most common types of weak copyleft are the GNU Lesser General Public License (or LGPL), the Perl foundation Version 2.0 of the Artistic License and the Mozilla Public License (or MPL).
  • On the other hand, the strong copyleft licenses mandate that any distributed software that links or contains such code has to be licensed under compatible licenses. For this reason, the strong copyleft licenses receive the appellation of “viral”. The most used strong copyleft licenses are the GNU General Public License (or GPL) and the Sleepycat License from Sleepycat Software (now part of Oracle). As a matter of fact, the most of the software under strong copyleft licenses is under the GPL license.

Important aspects about copyleft

Although the copyleft has many advantages and a few disadvantages, there are some aspects that should be considered.

There are still certain business models that are incompatible with copyleft licenses. The free software model is very different from the traditional private one: people aren’t still used to this model, which is hard to understand without a minimum previous formation. Unfortunately, a lot of users and software producers aren’t taking advantage of the copyleft licenses due to this lack of formation. The copyleft supporters must finish with the belief that using free software licenses will reduce his profit.

An example of these differences is the cost model. It is easy to understand that in the case of free software there are usually no license acquisition costs. Nevertheless, there still are a lot of other costs that are influencing each particular circumstance. These costs are there, although the calculation models used by private software are not in the free software.

Another interesting aspect is the way that the projects are performed. In the private system, the knowledge usually comes from paid professionals, while in the free software system the mechanisms to achieve quality are usually derived from the voluntary collaboration of the people.

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