Materials within public domain are works where no permission is needed to copy or use them. A work is generally considered to be within the public domain if it is ineligible for copyright protection or its copyright has expired. These works can be used freely and without permission or paying royalties.
Types of works that are generally not eligible for federal copyright protection:
In the U.S., "the length of copyright protection depends on several factors. Generally, for most works created after 1978, protection lasts for the life of the author plus 70 years. For anonymous works, pseudonymous works, or works made for hire, the copyright term is 95 years from the year of first publication or 120 years from creation, whichever comes first." - U.S. Copyright Office
Aside from using what you've learned here, there aren't clear signs about what does or doesn't fall into the public domain. However, you may see a symbol used in association with Creative Commons to identify works with no known copyright:
Every year, new works enter the public domain. Check out the SPARC 2024 Public Domain Day Celebration to learn more. Continue below to see where to find materials in the public domain.
https://copyright.universityofcalifornia.edu/use/public-domain.html CC BY-NC 4.0
https://www.copyright.gov/history/copyright-exhibit/lifecycle/
