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Straight up: I've never heard of the term 'copyleft' before. The fact that I get that little red squiggle under it tells me it's not really a word. Concisely, my question is, what does it mean to you?


I saw it tagged on a few questions so I read the tag description:

For questions about the copyleft concept (also known as Share-Alike), a concept that promotes or enforces the use of the same or compatible license for derived works.

And I read the Wikipedia page on it. My understanding is that copyleft has attributes of Share-Alike but the author must disclose any other terms alongside it.

To me, it seems like a bit of a useless term.

Why describe it as copyleft if you need to add a licence to it anyway? Would you not be better off describing it as copyright with already-agreed-upon licenses? Otherwise it's only another word for Share-Alike.

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    A quick note on "it's not really a word" - words are not defined by a committee of people in suits adding them to a big list, they are created by anybody, and spread if they prove useful. "Copyleft", for instance, has been in wide use in particular communities since the 1980s, with a well-defined meaning, so is very much a "real word" in that sense. – IMSoP Feb 8 '16 at 15:06
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    In all honesty it's hard to choose which answer to mark as accepted; they all add something. So, if you're reading this question in the future, be sure to read all the answers.! – jaunt Feb 13 '16 at 13:42
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Copyleft is actually a term coined by Richard Stallman (also called RMS) who is the pioneer of the Free Software Movement which ultimately resulted in today's world where people take using FOSS software so much for granted.

Stallman originally used this term to distinguish his way of free software licensing (GNU GPL/LGPL) from the other Copyright licenses which are typically designed by proprietary companies to keep the holding rights of a software to themselves by not disclosing the source.

You see, freedom to users is the major objective of FSF movement and it doesn't have anything to do with zero cost or free as in beer, it refers to free as in freedom, which is more expressed by the term libre than free.

The other camp in the FOSS world, by contrast, prefers to use the term permissive instead of copyleft. This camp favors software licenses like MIT, Apache, BSD, etc. which generally grant all usage rights along with the code. This second camp also prefers the term open source to free software. The reason is that software innovation resulting from source code being openly developed is a far more important objective for this camp than some liberal ideology.

Regardless, both these camps continue to contribute their efforts in the FOSS world today and to an extent, respect each other's views.

EDIT

To elaborate a bit on the legal implications, whilst both permissive and copyleft licenses grant their users freedom along with the code, there is a difference between these two kinds of freedom. Permissive licenses grant freedom to do just about anything with the source code to that one particular user. Copyleft licenses by contrast, tries to secure freedom to it's entire userbase by restricting that particular user's ability to make said source code proprietary or closed. It achieves this by mandating that any further modification to the copyleft licensed source should be distributed on a similar copyleft license terms. This mandate usually gives the incorrect impression to the initial observer that copyleft licenses are restrictive or "viral" in nature, but a detailed analysis reveals that its not true. Copyleft just tries to secure maximum freedom for the entire community or the user base, not just one user.

As I described in one of the comments below, the difference between permissive and copyleft is similar to the one between dictatorship and democracy. Rulers in both kinds of rule usually have the freedom to do anything, but democracy has some restrictions on the ruler since it tries to secure freedom to the entire masses, not just that one dictator.

EDIT-2

From the above analogy, I don't want to imply any negativity about the word dictator here, nor do I want to send a message that I'm in favor of copyleft licenses and against the permissive ones. In fact, dictatorship itself is a neutral word in Political Science, but only due to actions of a few bad rulers, its usage has acquired a negative connotation. Dictators can be good too (like BDFL Linus Torvalds). Similarly, permissive licensed software, in the right hands could bring out the best for everyone.

However, I do think GPL is a good thing because it prevents the appleization of software, but Apple positively surprised us today by taking a strong stance in favor of its users. Tomorrow, it might even go ahead and open-source the OSX stack (though I don't think it that likely in the near future).

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    @curiousdannii I think you are confusing user with programmer. A user of a copyleft license is most benefited and can do everything that a user of permissive license can, except this one thing: The user can't make it proprietary. I think, this ensures more freedom to the overall user-base (not talking of that one particular user). – Prahlad Yeri Feb 8 '16 at 1:06
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    True, but the freedom you are talking about is "freedom to turn an open-sourced code into a closed-sourced proprietary one" which I don't think is a good freedom, because it restricts the freedom of all users downstream of that proprietary software. It is like saying dictatorship is a much freer kind of rule because it allows more freedom to one particular user (or person). But that freedom comes at the cost of zero-freedom to everyone whom the dictator controls. – Prahlad Yeri Feb 8 '16 at 1:22
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    @MichaelT That just feels like nitpicking at the words. Practically speaking, the rights are given away - you provide an irrevocable licence on that content once you publish or distribute it. As long as other's follow the rights that you have relinquished, or given to them, you can't revoke that licence. No one said that they are taking the copyright from you, but we're saying that you are giving or providing us rights. The two sayings are essentially synonymous. – Zizouz212 Feb 8 '16 at 2:38
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    I'm going to mention that I really take further exception to the comparison of BSD licenses to dictators and the GPL to democracy. This type of opinionated and leading language reflects poorly on the goals of FOS as a whole. Such an analogy is at best misleading and flawed. – user756 Feb 9 '16 at 0:37
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    @PrahladYeri those analogies don't help newcomers at all. They make it more confusing, by suggesting that copyleft is somehow better and not wrong/evil (like those ebil permissive licenses). If anything, copyleft is more dictatorlike because it imposes far more conditions on derivative users than permissive licenses do. Which is a reason that is a poor analogy - it's actually suggesting the opposite meaning. Copyleft results in a far more strict set of conditions users who want to incorporate copyleft-licensed code must follow than a permissive license does. – enderland Feb 9 '16 at 2:03
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Copyleft is a general idea, not a specific license. It's used for talking about licenses. It means the general idea of using copyright to ensure open redistribution. Since copyright is usually used for the opposite of that, copyleft is copyright "turned on its head", thus the punning name.

As for why people would use that term instead of "Share-Alike":

  1. Copyleft came first, by at least 15 years.
  2. If you use the term "Share-Alike" people are likely to think that you're talking about Creative Commons specifically.
8

Prahlad provides an excellent answer on the origin of the term, but doesn't expand that much on the legal implications of using the term. In summary, it forces any modifications to copyleft works to also be released under the same terms.

First off, copyleft has no legal significance.

This means that if you were to say that a project is copyleft, it doesn't mean anything. Under the law, that statement isn't clear, and can't have any meaning, since copyleft is never defined under the law. Think of it as a term in popular culture: people use it, but it doesn't really mean much.

Second, there is no "one-size-fits-all" kind of copyleft.

Let's look at a few different "copyleft" licences: most notably, the Mozilla Public Licence, and the GNU General Public Licence. These are both "copyleft" licences - what's the difference?

A simple definition of copyleft is that changes will be released under the same licence. With the Mozilla licence, you can make your own projects under the Mozilla-licenced code, and your own code can be licenced under whatever.

With the GPL however, even if you make a function call to something under the GPL, your code has to be under the GPL too. If you do something like this:

GPL_Licensed_Code.do_something();

Your code has to be under the GPL, because yo've now created a link to that code.

Copyleft and Share-Alike are basically synonyms.

The tag wiki excerpt, as you noted, says that the two terms are basically synonyms. We used to have different tags for both concepts, but realized that the question that the tags were applied to utilized both tags, and tried to convey the same meaning. There is a good key difference to note:

Copyleft is used to describe source code.
Share-Alike is used to describe works of art.

In fact, copyleft was used as a term originating with the Free Software Foundation, whereas share-alike was used in the Creative Commons family of licences.

Fundamentally, they are the same, but the two terms are applied to different works.

Why describe it as Copyleft if you need to add a licence to it anyway? Would you not be better off describing it as copyright with already-agreed-upon licences?

Now that we've determined that there are different kinds of copyleft, and that copyleft is not a legal term, we can now answer the overall question.

Licences often provide a series of different terms, generally providing rights that govern patents, to attribution, to disclaimers of warranties and liabilities. Copyleft is just a 'popular culture' term: it's just used to simplify most of this. But it can't be used as a substitute for a licence.

Concisely, we just use copyleft to provide a general summary of a key point of the licensing terms, those affecting distribution. Aside from that, it's up to the legalese in the licences.

  • >"First off, copyleft has no legal significance." Not copyleft in particular, but GPL/LGPL does have the legal significance and the word copyleft license generally refers only to those two, isn't it? – Prahlad Yeri Feb 8 '16 at 1:15
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    @PrahladYeri The licence has significance, but not the term. The Mozilla Public Licence also is known as a copyleft licence. It's important not to conflate copyleft as a term, and as if it were an individual licence. – Zizouz212 Feb 8 '16 at 1:36
  • "Second, we can't call things copyleft because there are different kinds of copyleft." - That's like saying we can't call things 'animals' because there are different kinds of animals. Really no idea what you meant here. – Nye Feb 8 '16 at 18:59
  • @Nye Reading back on that, I can't even tell what I meant myself... :/ Is that edit better? – Zizouz212 Feb 8 '16 at 22:20
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    The "use GPLed code makes the user GPL" is a contentious issue. FSF insists it is, but that really depends on the (legally tenuously defined) meaning of "derivative work". – vonbrand Feb 9 '16 at 0:37
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Usually unless otherwise stated, by default, copyright law does reserve "all rights" for the holder of the copyright. Copyleft is the concept of using copyright law, turning it around to being friendly for distribution rather than maximizing the control for the copyright holder. As all rights are usually reserved, also the rights to specifically (re-)direct those rights are built into copy-right. There exist many different philosophies of how this can and should be used.


Creative Commons (CC) is one famous initiative to encourage open distribution in various contexts. There exist many different types of CC licences. Attribution (BY) meaning the author wants to reserve the right to be known as the author and that no one else can claim to have created the work. Share-Alike (SA) means everyone making derivatives of the work must let those derivatives remain shareable. Non-Commercial (NC) means anyone making money including the copyrighted material would not be allowed.


For computer softwares, there exist many different licenses. MIT, Berkley, Apache, GPL (1,2 and 3) are some well-known ones. The details are best learned for each of these separately. Some of the most important philosophical differences between these include the ability for a derivative work to have a new license or not. Some say license for redistribution should be required to remain the same, and some that the user should be able to change it.


Also it is possible to release a work under multiple licenses, for instance one which maximizes distribution, but does not allow commercialization for example educational and promotional purposes and one which allows licensing for commercialization but then would need to contact the author for the commercial contract.

  • It really doesn't answer the question of "what does copyleft mean". – vonbrand Feb 8 '16 at 2:34
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    This answer has a lot of information, and while that's not a bad thing, I don't think the extra information here really adds to the main answer that is provided in your first paragraph. Aside from that, good stuff! :) – Zizouz212 Feb 8 '16 at 2:35

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