Two examples of material in the public domain are United States government works, and writings for which their copyright has expired. Work created by the federal government is in the public domain (but not work created by state of local governments). Expired works in the public domain in the United States, beginning January 1, 2023, include everything first published or released there prior to before January 1, 1928. Laws vary by country, but in most of the world, works whose authors died at least 50, 70, or 100 years ago are in the public domain. (The 1928 cut-off date applies only to the U.S. This means foreign works first published before 1928 are in the public domain in the U.S., but may still be copyrighted outside the U.S.; The claim that "pre-1928 works are in the public domain" is correct only for published works; unpublished works are under federal copyright for at least the life of the author plus 70 years.)
Fair use is not needed when copying material that is in the public domain because there is no copyright holder. However, physical access to the works may be controlled by a museum that makes users promise not to make any reproductions. Texts and images published in the U.S. before January 1, 1928 are in the public domain. (Because of legislation passed in 1998, no new works fell into the public domain until 2022, when the copyright in works published in 1926 expired. In 2020, works published in 1924 expired, and so on. For works published after 1977, if the work was written by a single author, the copyright will not expire until 70 years after the author’s death.) Before the Copyright Act of 1976, copyright expired after 28 years and had to be renewed for an additional 28-year term; items published prior to 1964 that were not renewed are in the public domain. Items that were published in the USA before 1989 without a copyright notice are, with a few exceptions, in the public domain. Works originally published in the U.S. after 2002 (with or without copyright notice or registration) are protected until 70 years after the author's death. 
Through the Berne Convention Implementation Act, (effective March 1, 1989) copyright is automatic; no registration is needed and it is not necessary to display a © notice to be copyright protected.
A very useful chart (substitute 1928 for 1923, as of Jan. 1, 2023): Copyright Term and the Public Domain in the United States. by Peter B. Hirtle. (Last updated 3 January 2014). Cornell Copyright Information Center 
A chart of the copyright durations of US works, by Jappalang.
On 1 March 1989, the U.S. Berne Convention Implementation Act of 1988 was enacted, and the U.S. Senate ratified the treaty, making the United States a party to the Berne Convention. Both the Senate and the House of Representatives passed legislation to bring United States laws into conformity with the Bern treaty. Summary of the Berne Convention for the Protection of Literary and Artistic Works. In the United States, determining whether a work has entered the public domain or is still under copyright can be quite complex, primarily because copyright terms have been extended multiple times and in different ways—shifting over the course of the 20th century from a fixed-term based on first publication, with a possible renewal term, to a term extending to fifty, then seventy, years after the death of the author. US Copyright Office, Copyright Basics.
- The Public Domain Review
- How Can I Tell Whether a Book Can Go Online? Guidance from the University of Pennsylvania Online Books project.
- FAIR USE - PUBLIC DOMAIN An introduction to copyright terms and a discussion of Bridgeman vs Corel.
- Ongoing list of books in the public domain
- US Copyright Laws; a summary.
- WHEN U.S. WORKS PASS INTO THE PUBLIC DOMAIN.
- Public Domain for 2021
- Superb, growing resource of public domain images
- ↑ Welcome to the Public Domain.
- ↑ For a list of renewals see the Project Gutenberg collections at , which cover everything except music.
- ↑ Copyright Term and the Public Domain in the United States.
- ↑ WHEN U.S. WORKS PASS INTO THE PUBLIC DOMAIN.
- ↑ Definitions. U.S. Copyright Office.
- ↑ Cornell Copyright Information Center.