http://digital.lib.uiowa.edu/u?/ictcs,5310

Image by anarres at openclipart.org

Copyrighted material can be used freely if it is in the public domain, which is the case where the work is no longer protected by copyright or never met the requirements for copyright protection in the first place. Because works in the public domain are no longer protected by copyright, they may be freely used without permission.

Determining whether something is truly in the public domain can be difficult. This chart from Cornell University is a commonly used reference source that can help.

The Copyright Clearance Center is a site to get permission to reproduce copyrighted works, when necessary. It can also help you determine whether fair use applies.

Section 107 of the U.S. Copyright law provides for the use of copyrighted materials under certain circumstances defined as “fair use.” The law lists required criteria to  determine if a particular use can be considered fair use under the law.

1. the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes
2. the nature of the copyrighted work
3. the amount and substantiality of the portion used in relation to the copyrighted work as a whole
4. the effect of the use upon the potential market for or value of the copyrighted work.

How do I know if it's fair use? 

These tools can help: