Copyleft - Reciprocity

Reciprocity

Copyleft can be characterized as a copyright licensing scheme in which an author surrenders some but not all rights under copyright law. Instead of allowing a work to fall completely into the public domain (where no ownership of copyright is claimed), copyleft allows an author to impose some restrictions on those who want to engage in activities that would more usually be reserved by the copyright holder. Under copyleft, derived works may be produced provided they are released under the compatible copyleft scheme.

The underlying principle is that one benefits freely from the work of others but any modifications one makes must be released under compatible terms. For this reason some copyleft licenses are also known as reciprocal licenses, they have also been described as "viral" due to their self-perpetuating terms. Under fair use, however, the copyleft license may be superseded, just like regular copyrights. Therefore, any person utilizing a copyleft-licensed source for their own work is free to choose any other license provided they meet the fair use standard.

While copyright law gives software authors control over copying, distribution and modification of their works, the goal of copyleft is to give all users of the software the freedom to carry out these activities. In this way, copyleft licenses are distinct from other types of free software licenses, which do not guarantee that all "downstream" recipients of the program receive these rights, or the source code needed to make them effective. In particular, permissive free software licenses such as BSD allow re-distributors to remove some or all these rights, and do not require the distribution of source code.