Creative Commons and Public Domain: Public Domain

Any work that belongs to the Public Domain is not covered by copyright law. In other words, the work belongs to the public as a whole and everyone has a right to use or adapt the work as they choose. Works inside the Public Domain fall under one of the following categories:

• content that is no longer protected by copyright, either because the copyright has expired or a renewal was not properly filed; or
• content that was originally intended for the Public Domain, such as the CC0 License from Creative Commons

Project Gutenberg is an online collection of books that had copyright protection in the past but are now part of the Public Domain. Since these books belong to everyone, anyone can access the content and make adaptations to it without fear of violating copyright law. 

For more information about the Public Domain, see "Welcome to the Public Domain." Copyright & Fair Use, Stanford University Libraries,
fairuse.stanford.edu/overview/public-domain/welcome/.

"What is Public Domain?". YouTube, uploaded by U. S. Copyright Office, 18 December 2019, https://youtu.be/PMp_-OX15Jc.