Any work assigned a Creative Commons public domain license by its creator.
All works published in the United States before 1923.
All works published in the United States between 1923 and 1977 without a copyright notice.
All works published in the United States between 1923 and 1963 that did not obtain a copyright renewal.
All works published in the United States between 1978 and March 1, 1989 that did not obtain copyright renewal five years after creation.
If you are unsure about whether or not a work is in the public domain, check this guide created by the US Copyright Office to help you determine the status.
How to Investigate the Copyright Status of a Work