When talking about copyleft, copyright could not be missed. Copyright is about intellectual property. Basically any type of creative or artistic work can be copyrighted. It could be literature, musical compositions, or software. Rights can include but are not limited to distribution and modification. Copyright usually expires after 50 years of the author’s death, which makes it fall into the Public Domain.
Copyleft is always compared to copyright. However, it is not a situation-what is left in copyright goes to copy left. Copyleft is closely related to software.
Copyleft is a general method for making a program (or other work) free, and requiring all modified and extended versions of the program to be free as well. --GNU.org
Copyleft and licenses
Copyleft licenses require information for reproducing and modifying the work be available to recipients of the binaries . The source code files usually contain a copy of the license terms and acknowledge the authors . The GNU General Public License (GPL), written by Richard Stallman , was the first software copyleft license. Creative Commons , a non-profit organization founded by Lawrence Lessig , provides a similar license-provision condition called share-alike .
Copyleft cannot be simply understood as what is left in copyright. There are three aspects that you should know according to Joel Lee .
Copyleft is about user freedom. Copyleft isn’t about abolishing copyrights. The core concept of copyleft is that users should have the right to freely use, copy, modify, and distribute works however they want, with one crucial clause: all derivative works must offer the same freedoms to users.
Copyleft is more than permissions. The most notable aspect of copyleft licenses is that they require users to distribute derivative works under a license that offers the same rights as the original work. This is a “share-alike” clause mentioned above. This made BSD and MIT licenses not copyleft licenses.
Copyleft is not always free . But once you pay for a copyleft work, you’re free to modify and distribute it as long as you maintain the same copyleft freedom in the derived work. Creative Commons Attribution-ShareAlike license ( CC BY-SA) and Creative Commons Attribution-NonCommercial-ShareAlike license ( CC BY-NC-SA) are two copyleft licenses that can be used to distributing the work.
In a word, copyleft is a philosophy and it is very difficult to make money using a copyleft license. However, those who believe in it will carry on the freedoms of users.