Public Domain works are not protected by copyright law and they may therefore be legally used without permission.
A work may enter the public domain when/if:
- Its copyright protection expires
- It fails to meet requirements for copyright protection (e.g., Federal government publications, for the most part)
- It predates copyright law
- The author chooses to release it to the public domain
More info: Copyright Term and the Public Domain in the United States. (January 2013 Cornell University) and Is it in the Public Domain?, a series of flowcharts from Samuelson's clinic at UC Berkeley Law School.
While certain works may be in the public domain, expressions or versions of that work may be under copyright protection.
- For example, Mary Cassatt's paintings may be in the public domain, but photographs of those paintings may be copyrighted.
Read and comply with the use restrictions for specific image sources and always cite images someone else created.
[Image: Lee, Russell, Sonoma County, California.
The noon collection of eggs..., 1942.
Library of Congress.]