No permission or license is required for a work truly in the public domain, such as one with an expired copyright; such a work may be copied at will. Public domain equivalent licenses exist because some legal jurisdictions[which?] do not provide for authors to voluntarily place their work in the public domain, but do allow them to grant arbitrarily broad rights in the work to the public.
The licensing process also allows authors, particularly software authors, the opportunity to explicitly deny any implied warranty that might give someone a basis for legal action against them. While there is no universally agreed-upon license, several licenses aim to grant the same rights that would apply to a work in the public domain.
In 2000, the "Do What the Fuck You Want To Public License" (WTFPL) was released as a public-domain-equivalent license for software. It is distinguished among software licenses by its informal style and lack of a warrantydisclaimer. In 2016, according to Black Duck Software,[note 1] the WTFPL was used by less than 1% of FOSS projects.
The Unlicense software license, published around 2010, offers a public-domain waiver text with a fall-back public-domain-like license, inspired by permissive licenses but without an attribution clause. In 2015 GitHub reported that approximately 102,000 of their 5.1 million licensed projects, or 2%, use the Unlicense.[note 3]
In the free-software community, there has been some controversy over whether a public domain dedication constitutes a valid open-source license. In 2004, lawyer Lawrence Rosen argued in the essay "Why the public domain isn't a license" that software could not truly be given into public domain, a position that faced opposition by Daniel J. Bernstein and others. In 2012, Rosen changed his mind, accepted CC0 as an open-source license, and admitted that, contrary to his previous claims, copyright can be waived away.
In February 2012, when the CC0 license was submitted to the Open Source Initiative for approval, controversy arose over a clause which excluded any relevant patents held by the copyright holder from the scope of the license. This clause was added with scientific data in mind rather than software, but some members of the OSI believed it could weaken users' defenses against software patents. As a result, Creative Commons withdrew their submission, and the license is not currently approved by the OSI.
In June 2020, following a request for legacy approval, OSI formally recognized the Unlicense as an approved license meeting the OSD.
Google does not allow its employees to contribute to projects under public domain equivalent licenses like the Unlicense and CC0, while allowing contributions to 0BSD licensed and US government PD projects.
^1. MIT License: 26%; 2. GNU General Public License (GPL) 2.0: 21%; 3. Apache License 2.0: 16%; 4. GNU General Public License (GPL) 3.0: 9%; 5. BSD License 2.0 (3-clause, New or Revised) License: 6%; 6. GNU Lesser General Public License (LGPL) 2.1: 4%; 7. Artistic License (Perl): 4%; 8. GNU Lesser General Public License (LGPL) 3.0: 2%; 9. ISC License: 2%; 10. Microsoft Public License: 2%; 11. Eclipse Public License (EPL): 2%; 12. Code Project Open License 1.02: 1%; 13. Mozilla Public License (MPL) 1.1: < 1%; 14. Simplified BSD License (BSD): < 1%; 15. Common Development and Distribution License (CDDL): < 1%; 16. GNU Affero General Public License v3 or later: < 1%; 17. Microsoft Reciprocal License: < 1%; 18. Sun GPL With Classpath Exception v2.0: < 1%; 19. DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE: < 1%; 20: CDDL-1.1: < 1%
^In the above bar-chart I have counted GPL and its different versions as one family, and I did the same with LGPL too. From this diagram it is very much clear that the MIT License is the most used license, with a total number of use case of 2706.Therefore comes GPL (i.e. GNU General Public License) and its different versions, BSD, LGPL (i.e. GNU Lesser General Public License) and its different versions, ASL (i.e. Apache Software License) family, MPL (i.e. Mozilla Public License). Apart from these licenses there are projects who has submitted themselves in to Public Domain and that number is 137.
^Lawrence Rosen (2004-05-25). "Why the public domain isn't a license". rosenlaw.com. Retrieved 2016-02-22.
^Placing documents into the public domain by Daniel J. Bernstein on cr.yp.to: "Most rights can be voluntarily abandoned ('waived') by the owner of the rights. Legislators can go to extra effort to create rights that can't be abandoned, but usually they don't do this. In particular, you can voluntarily abandon your United States copyrights: 'It is well settled that rights gained under the Copyright Act may be abandoned. But abandonment of a right must be manifested by some overt act indicating an intention to abandon that right. See Hampton v. Paramount Pictures Corp., 279 F.2d 100, 104 (9th Cir. 1960).'" (2004).
^Lawrence Rosen (2012-03-08). "(License-review) (License-discuss) CC0 incompliant with OSD on patents, (was: MXM compared to CC0)". opensource.org. Archived from the original on 2016-03-12. Retrieved 2016-02-22. The case you referenced in your email, Hampton v. Paramount Pictures, 279 F.2d 100 (9th Cir. Cal. 1960), stands for the proposition that, at least in the Ninth Circuit, a person can indeed abandon his copyrights (counter to what I wrote in my article) – but it takes the equivalent of a manifest license to do so. :-) [...] For the record, I have already voted +1 to approve the CC0 public domain dedication and fallback license as OSD compliant. I admit that I have argued for years against the "public domain" as an open source license, but in retrospect, considering the minimal risk to developers and users relying on such software and the evident popularity of that "license", I changed my mind. One can't stand in the way of a fire hose of free public domain software, even if it doesn't come with a better FOSS license that I trust more.
^"Using CC0 for public domain software". Creative Commons. April 15, 2011. Retrieved May 10, 2011.
^"Various Licenses and Comments about Them". GNU Project. Retrieved April 4, 2015.
^Carl Boettiger. "OSI recognition for Creative Commons Zero License?". In the Open Source Initiative Licence review mailing list. opensource.org. Archived from the original on September 26, 2013. Retrieved February 1, 2012.
^Christopher Allan Webber. "CC withdrawl [sic] of CC0 from OSI process". In the Open Source Initiative Licence review mailing list. Archived from the original on 2015-09-06. Retrieved February 24, 2012.
^Chestek, Pamela (June 16, 2020). "[License-review] Request for legacy approval: The Unlicense". Archived from the original on September 8, 2020.