Copy Left

CopyLeft: All rights reversed.

CopyLeft is a license that permits people to freely copy, modify and redistribute software so long as they do not keep others from also having the right to freely copy, modify, and redistribute the software. You can actually sell CopyLeft software (haven't you seen box sets of Linux at the computer store?), but you must also offer the source for free, either with the product or available for free (or cost of copying/shipping) to anyone on request.

Its purpose is essentially to make software irrevocably FreeAsInSpeech.

Some people are suspicious of CopyLeft because it is more or less infectious - you can't directly include CopyLefted code in your proprietary program without your proprietary program falling under the CopyLeft license - thus becoming non-proprietary. If, on the other hand, you're interested in keeping your development FreeSoftware, CopyLeft is the bee's knees.

One way to look at CopyLeft is as a way to prevent the TragedyOfTheCommons.

It's more a way to prevent the FreeRiderProblem: nobody can take the community's software and add things to it that they don't share with the community.

You can read more about it here: http://www.gnu.org/copyleft/copyleft.html

Examples of CopyLeft licenses are the GnuGeneralPublicLicense and the GnuLesserGeneralPublicLicense.

A Taxonomy

Copyleft Extremist: Extreme form of Copyleftist. Believes all information should be under CopyLeft, and casts aspersions on the motives of anyone who produces anything else from the most illegal of monopolists to the most altruistic creators of public-domain information.
Copyleftist: Milder form of Copyleft Extremist. Chooses CopyLeft for all personal creations, and believes the world would be a better place if everyone else did the same. Another possible interpretation for the term "copyleftist" is a licensing clause which forbids use of the software by oppressive, racist, militaristic, environmentally unsound and related users and organizations. For example, refer to this discussion: http://www.tc.umn.edu/~brams006/copyleftist.html It should that the OpenSourceInitiative? specifically excludes such licenses from the definition of OpenSource - to qualify as OpenSource, the software must be usable by anyone regardless of purpose. Such clauses are on shaky legal ground anyways. Additional problems might, theoretically, occur if one's OpenSource software were used (for example) to monitor citizens, compromise privacy, wage or model war, etc. Is complete freedom of use necessarily the most laudable goal, or shall the OpenSource movement strive for a sense of ethics and become a force of positive social change? One might draw parallels with unbridled free speech. It is recognized that harassment, libel, slander, terroristic threats, falsely yelling 'fire' in a crowded theater, propaganda, and so forth represent stages at which free speech can become self-defeating or perhaps not even as important as fair speech ('free' vs fair trade also).
Copyleft Moderate: Chooses to use and publish CopyLeft information whenever appropriate for personal and business goals. Has no problem with other forms of copyrighted information. Most likely opposed to software patents and rampant extension of copyright term.

The less suspicious opposition to copyleft contrasts it to other style of free software license applied to the BSD and X11 distributions, and points out the rhetoric about copyleft protecting freedoms is inconsistent, in that use of copyleft constrains what you can do with the source. Untrue. GPL, for example, only restricts distribution - while it even allows linking with proprietary software, as long as you do not distribute the result to third parties.

It is interesting to note that the philosopher behind copyleft has clarified his position over the years after many people pointed out how it restricted the freedom of programmers to earn a living creating proprietary software. Now RichardStallman is very clear on the fact that CopyLeft is to protect user's freedoms, not necessarily programmers.

...the rhetoric about copyleft protecting freedoms is inconsistent, in that use of copyleft constrains what you can do with the source.

"Inconsistent" is a very strong word. Do laws promoting free speech actually work against freedom? After all, they keep me from suppressing your free speech, thereby making me less free. Is that "inconsistent"?

As far as "constraining what you can do with the source": you can do anything programmatic you want with your source. You can do anything programmatic you want with CopyLeft source. You just can't sell the CopyLeft software as if you own it.

The statement above is not completely true. For example, I do not believe that there is any legal way to combine copyleft software with public-domain software, because once something has been placed in the public domain, it cannot be copyrighted. This fact makes it impossible to impose any licensing conditions, including the copyleft license, on public-domain software.

The above example is wrong. Works which include copyrighted material (including CopyLeft licensed material) and public domain material can be licensed as if the public domain material was not there. This means GPL+public domain = GPL is legal. That is what public domain means; material which imposes no licensing conditions, including not imposing the condition that no conditions can be applied to it. - JesseFW

If there is no legal way to combine copyleft software and public domain software, then there is also no legal way to combine public-domain software with proprietary software. So all the proprietary projects that have incorporated, for example, the reference implementations of the Blowfish, Twofish, and Rijndael encryption algorithms, are illegal.

Not true. For all X, the legality of combining X with public-domain software depends entirely on the nature of X. If X is GPL software, the difficulty arises that the GPL requires anything I distribute together with X be restricted by the GPL, and there is no way to impose such a restriction on public-domain information. In particular, if you distribute public-domain software as part of your proprietary product, you cannot legally restrict anyone else's use of that part of your product. This state of affairs is incompatible with the GPL, because it means that you cannot compel people to whom you give your product to give the public-domain parts to other parties. There is no reason to suppose that other proprietary projects have similar difficulties.

Actually, you can restrict the use of the public domain part of your product by relicensing it under the GPL. That doesn't mean the code is no longer public domain, but your distribution of the code is.

To clear up another popular misconception: anyone is free to sell CopyLeft software for whatever price they can get. But you can't restrict the buyers' rights in regard to reuse, modification, and distribution, and you must adhere to whatever conditions are in the license in regard to providing source along with executables. It's true that the "free availability" of CopyLeft software means you may not be able to charge much for it nor force customers to pay, but there is nothing about a general CopyLeft license that restricts sales.

The free speech analogy makes the right point: CopyLeft gives you the freedom to do anything except take away someone else's freedom by adding new restrictions (either directly by making a proprietary version, or indirectly changing to a license that would allow other people to make proprietary versions). How this affects selling CopyLeft software is an interesting but separate topic.

Just as with any other distribution system, Copyleft has its advantages and disadvantages. The marketplace of software is in the process of deciding a very important question: Does the good outweigh the bad? Only time will tell. -- RyanKelly

Is it the case that someone can take CopyLeft software and represent it as their own creation? In other words, is copyleft software not only free, but also authorless?

No, at least not legally. Most CopyLeft licenses, including the GnuGeneralPublicLicense, require that any derived works give proper credit to the work they come from. In addition, stripping off copyright notices and the like is a major no-no.