It is not always easy to identify whether a work is in the public domain. Creative Commons also has a tool called the Public Domain Mark, which can label works whose copyright has expired everywhere in the world. While the mark does not have legal effect, it ideally helps re-users identify works in the worldwide public domain.
With millions of creative works whose copyright has expired—and many more added regularly with tools like the CC0 Public Domain Dedication, the public domain is a vast treasure trove of content.
Despite the expansive reach of copyright, there is still a rich (and growing) public domain full of works which are free from copyright. Works enter the public domain when:
Depending on the scope and duration of copyright protection in a particular country where the work is used, you can do almost anything with a work in the public domain. Depending on the country, for example, a work in the public domain may still be covered by moral rights that last beyond the duration of copyright. It’s also possible that a work is in the public domain in one country, but is still under copyright in another country. This means you may not be able to use the work freely where copyright still applies.
A public domain work may still be subject to other intellectual property restrictions. For example, a public domain story may have a trademarked brand on the cover associated with the publisher of the book. Trademark protection is independent of copyright protection, and may still exist even though the work is in the public domain as a matter of copyright. Also, once a creator uses a public domain work to turn it into a new work, the creator will have copyright on the portions of their new work that are original to them. As an example, the creator of a film adaptation based on a public domain novel will have copyright protection over the film but not the underlying public domain novel.
There are many benefits to identifying and giving credit to the original creator, even after her work has entered the public domain. Creative Commons has created public domain guidelines that can be used by communities to create their own norms.