Copyright Timeline: A History of Copyright in the United States
Copyright Basics video -- Copyright Clearance Center
Works not protected by copyright are considered to be in the public domain and may be reproduced without permission. As of January 1, 2003, any work published or copyrighted prior to January 1, 1923 has expired by operation of the law and has fallen permanently into the public domain in the United States. Works by the U.S. Government are not eligible for U.S. copyright protection and are generally in the public domain.
Digital Copyright Slider: use this tool to help determine copyright duration.