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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.
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.
These tools can help: