Fair use of copyrighted material in the U.S. is explained in American University’s excellent Best Practices in Fair Use series. However, fair use covers only a limited set of uses for copyrighted works; public domain material can be used for any purpose.

As I understand it, here are the basic rules on what’s in the public domain [this is not legal advice]:

1. All works published up to and including 1922 are free to use. Just confirm the publication date.

2. Some works published from 1923 up to and including 1963 are free to use; the renewal records for books only are available here. During these dates, works were protected for 56 years, but you had to file a renewal after 28 years, which many people forgot to do. So up through the 1950s, a lot of material regularly passed into the public domain. If the work does not turn up in the search, that’s good–it means no renewal was filed, so you’re probably OK.

3. Very few works published from 1964 up to and including 1988 are free to use; the records are available here. The Library of Congress maintains complete online records for everything published from 1978 to the present. You can also search for works published from 1964 up to and including 1977, but be aware that the online records for those are incomplete. (Only renewals from those years are listed; you only have a record if someone still filed their 28-year renewal, even though the law had changed and they were no longer required to do so.)

4. No works published from 1989 onward are safe to use; with repeated term extensions, they may never become public domain. If you can’t make a case for fair use, you’ll always need permission from the rights holder. The rights holder might charge an expensive license fee, deny you permission, or just refuse to speak to you; an unscrupulous rights holder might also lie about what rights they actually own.

5. If you can’t find a clear answer, or for any important project, it may be worth contacting the publisher or paying for a Library of Congress card catalog search. This costs a shocking USD $165, but you do get an official-looking letter of confirmation if the work is in the public domain. By the way, this can be useful protection if your use of the work is going to end up on broadcast TV or something similar.

In all cases, when you run into obstacles, remember to check whether your use of copyrighted material might conceivably be covered by fair use rules. While the U.S. has some of the world’s strictest copyright laws, it also has among the world’s strongest fair use protections.

Unfortunately, even in cases where you’re technically in the right, distributors, broadcasters, ISPs, and content aggregators will often become frightened of legal trouble and refuse to touch your work. The Electronic Frontier Foundation provides legal advice for artists and defends people wrongly accused of infringement.