Subject areaRules, risk and compliance
Copyleft is the reversal of copyright devised by Richard Stallman in the 1980s: it uses the tools of copyright law to get the opposite effect.
The author keeps the rights and allows everyone to use, modify and redistribute the work, on one condition: whoever redistributes it, modified or not, must grant the same freedoms under the same terms. The name plays on right and left: from «all rights reserved» to «all rights left».
The practical consequence is that the work's freedom travels down the chain of derivative versions, and nobody can lock it inside a proprietary product. The best-known software example is the GPL; in Creative Commons licences the equivalent is the SA clause (Share Alike).
Copyleft differs from permissive licences, such as MIT or BSD, which only ask for credit to the author and leave whoever reuses the code free to close their own version.
An example
The case below is constructed, because the term does not yet appear in the articles. A university chemistry lab publishes a safety manual under CC BY-SA; a chemical company adapts it to its own plants and shares it with suppliers: the adapted version carries the same licence, so anyone can improve it in turn.
Enrico Giubertoni. Licensed under CC BY-NC 4.0: you may quote, translate and adapt this entry with the author’s name and a link, stating any changes. For commercial use, write to info@enricogiubertoni.com
Spotted an inaccuracy or something that needs updating? Write to info@enricogiubertoni.com and help make this glossary a better place.
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