The Public Domain consist of works that are available to use without permission.
There are many reasons a work might be in the public domain. A few common reasons are:
Common symbols for public domain works. Also look for works labeled as "no known copyright restrictions"
This site is intended for informational purposes only. Library staff members cannot give legal advice. For legal advice, you should contact an intellectual property attorney.
Meet with a research librarian to discuss a copyright question.
Remember: just because something is in the public domain doesn't mean you shouldn't cite it. Always give proper credit to your source!