The Public Domain is a state of belonging to the public as a whole and not being protected by copyright law. Works in the public domain are those for which copyright protection has expired, been forfeited or were inapplicable. They can be copied, distributed, performed and displayed without seeking permissions or applying an exception under copyright law. For example:
Forfeit: Copyright protection for Sita Sings the Blues has been forfeited by the creator Nina Paley in a public announcement updated in 2011. Authors can also forfeit their copyright in a work by utilizing the CC zero license. See Creative Commons for more.
Inapplicable: U.S. Federal Government publications are not granted copyright protection at all. All U.S. Federal Government publications fall immediately into the public domain. (note: publications made by third parties under government contract and by state governments (for example) are considered differently by the law and may be protected by copyright.)
The vast majority of the public domain is material for which copyright protection has expired. In the U.S., due to changes in copyright law over time, copyright is expired for material published over 95 years ago. Material published up until 1989, that did not adhere to the formalities required by copyright law at the time, like notice and registration, may also be in the public domain.
Unpublished materials, like diaries and letters, are protected for the life of the author plus 70 years regardless of their creation date.
Materials from other countries may be protected for different periods of time based on the timing and conditions of trade agreements between the country of creation and the country of use.
Digital slider that shows the possible copyright status of works created at different times.
For material published and registered for copyright between the 1923s and 1960s, continued copyright protection would require renewal with the US Copyright Office. There are also a few handy catalogs that can help you track down whether a work was registered or renewed (both registration and renewal could happen in the years preceding and following publication):
Stanford's Copyright Renewal Database is a searchable index of the copyright renewal records for books published in the US between 1923 and 1963. Note that the database includes only renewal records, not original registrations, and only Class A (book) renewals received by the US Copyright Office between 1950 and 1992.
Celebrating the Public Domain
The USF Libraries have an annual celebration of the material for which copyright has expired that is newly falling into the public domain. Each year, we have a community vote to determine which newly public domain materials from our print collections will be digitized and added to our Digital Collections online.