Apart from public domain, there are licenses that are considered public-domain-equivalent licenses and are very short licenses that keep no rights on the software.
People considering releasing a software as public domain or equivalent should take into account that releasing a code into the public domain or equivalent could bring legal issues to software cotributors and users, like patent-related claims from contributors or third parties.
Most FOSS projects go for permissive licenses instead on public domain or public-domain-equivalent licenses not by chance; they are seeking a minimal license that prevents the contributors or users being sued.
Public domain
Public domain would be at the top of permissiveness.
Anyone can modify and use a software under public domain without restrictions.
The concept of public domain varies within countries and jurisdictions, and some (like the moral right of recognized authorship) cannot be waived. An alternative to releasing software under public domain is using permissive licenses, like public-domain-equivalent licenses.
An example of software released as public domain is SQLite.
Public domain-equivalent licenses
European Union’s Joinup recommends the very permissive MIT over public domain-equivalent licenses for more standardization and good practice.
Public-domain-equivalent licenses featured on this post:
- The Unlicense
- Creative Commons Zero 1.0 Universal
The Unlicense
The Unlicense is a public-domain-equivalent license.
It is compatible with GNU GPL licenses.
FSF recommends to use CC0 or CC Zero license over the Unlicense, stating that:
CC0 also provides a public domain dedication with a fallback license, and is more thorough and mature than the Unlicense.
When someone applies the CC0 license to their work, they are essentially waiving all their copyright and related rights to the extent allowed by law. This dedication allows others to use the work for any purpose without any restrictions. However, legal systems and interpretations of public domain dedication can vary across countries, and there might be situations where the dedication is not fully effective in placing the work in the public domain.
To address this potential issue, CC0 includes a fallback license. This means that if the public domain dedication is not valid or not recognized in a particular jurisdiction, the fallback license acts as a backup, providing a more traditional copyright license with minimal restrictions. This ensures that the work remains as open and accessible as possible, even in jurisdictions where the public domain dedication might not be legally effective.
Creative Commons Zero 1.0 Universal
Creative Commons Zero, commonly abbreviated as CC0, is a public-domain-equivalent license aimed for artworks.
Creative Commons does not recommend to to release software with a CC0 license, and recommend to use a software-specific licenses. Their position is explained on this external link.
FSF does not recommend to use a license that gives up patent licenses, like CC0. As explained by FSF on this external link:
[CC0] For works of software it is not recommended, as CC0 has a term expressly stating it does not grant you any patent licenses.
Because of this lack of patent grant, we encourage you to be careful about using software under this license; you should first consider whether the licensor might want to sue you for patent infringement. If the developer is refusing users patent licenses, the program is in effect a trap for users and users should avoid the program.