Works in the Public Domain are not protected under copyright law and may be freely used by anyone!
Generally these include works published before 1923; those whose copyright terms have expired; or anything published by the U.S. federal government. Copyright terms vary depending upon the life of the author and whether or not the author is a corporation.
More about the Public Domain:
Copyright Term and the Public Domain in the United States
By Peter B. Hirtle
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Use this search tool at the United States Copyright Office for tracking down copyright records.
The Public Domain
University of Minnesota Copyright Services