There are certain requirements and restrictions on the use or interchange of works that we feel do not impede the essential freedom in our definition. These restrictions are described below.
Apart from these allowed restrictions, the license must not include clauses that limit essential freedoms. Especially, it must not specify any usage restrictions (such as prohibiting commercial use of the work, restricting use depending on political context, etc.).
Attribution protects the integrity of an original work, and provides credit and recognition for authors. A license may therefore require attribution of the author or authors, provided such attribution does not impede normal use of the work. For example, it would not be acceptable for the license to require a significantly more cumbersome method of attribution when a modified version of the licensed text is distributed.
Such an attribution should not imply an endorsement by the original authors of changes made by others. Licenses may place restrictions on the use of one author's trademarks in versions of the work which have been modified by others.
Transmission of freedoms
The license may include a clause, often called copyleft or share-alike, which ensures that derivative works themselves remain free works. To this effect, it can for example require that all derivative works are made available under the same free license as the original. Strong copyleft licenses have the broadest definition of derivative works - only permitting the modified versions of the work to be distributed with other Free Cultural Works. Weak Copyleft licenses require modified versions of the work to be distributed under the same license as the original work but allow it to be distributed with non-free cultural works - such as allowing photographs licensed under Free Culture licenses to be published with text which is not. (Licenses without copyleft/share-alike restrictions are called permissive or copyfree.)
Protection of freedoms
The license may include clauses that strive to further ensure that the work is a free work: for example, access to source code, or prohibition of technical measures restricting essential freedoms.
- Availability of source data: Where a final work has been obtained through the compilation or processing of design information or a source file or multiple source files, all underlying source data should be available alongside the work itself under the same conditions. This can be the score of a musical composition, the models used in a 3D scene, the data of a scientific publication, the drawings and parts list of a machine, or any other such information.
- Use of a free format: For digital files, the format in which the work is made available should not be a format that can only be read using a particular manufacturers programme. Formats should be documented and should not be protected by patents, unless a world-wide, unlimited and irrevocable royalty-free grant is given to make use of the patented technology. While non-free formats may sometimes be used for practical reasons, a free format copy means that the information will be accessible to everyone, for ever.
- No technical restrictions: The work must be available in a form where no technical measures are used to limit the freedoms enumerated above.
- No other restrictions or limitations: The work itself must not be covered by legal restrictions (patents, contracts, etc.) or limitations (such as privacy rights or being for non-commercial use only) which would impede the freedoms enumerated above. A work may make use of existing legal exemptions to copyright (in order to cite copyrighted works), though only the portions of it which are unambiguously free constitute a free work.
Restrictions which are not permissible
Apart from these allowed restrictions, the license must not include clauses that limit essential freedoms. Especially, it must not specify any usage restrictions
The freedom to make changes and to distribute derivatives -- one of the core freedom at the heart of the definition -- is clearly not available to use the users of works under licenses which block or restrict the creation of derivative works. As a result, no license that bars the creation of derivative works without permission can be considered a Free Culture license.
Licenses which only allow non-commercial use are not considered Free Culture licenses. This is because in practice there are many ways that Cultural works can be used and reused which would be considered commercial. Another problem is that there is no generally agreed definition of where the border line is between Commercial and Non Commercial uses with very many cases falling in the undefined area in between. In practice Share Alike or Copyleft clauses provide a restriction on commercial profits since any reuse making excessive profits will soon stimulate a bunch of copycats which will bring prices down while encouraging even wider distribution of the works - which is the objective of the free culture licenses.
Restrictions to limit use of works in support of causes which the original author abhors have been proposed at times. Licenses with these restrictions are not considered to be Free Culture licenses. In practice there are so many different causes, each of which has supporters so certain works can only be used with certain other works. This means any reuse must be subject of careful checking of exactly what restrictions apply.
3rd world only
Licenses for free distribution in poor countries only have been proposed. These are not considered Free Culture licenses.
The original BSD license included a clause requiring all advertising for a derivative work to include an acknowledgement of the contribution of the original author. With collaboratively produced works with many authors this would be a significant burden. Attribution requirements for Free Cultural Works should be simpler than this.