The public domain consists of those works whose copyright protection has expired, as well as those which never had copyright protection in the first place. According to the U.S. Copyright Office, "Works in the public domain may be used freely and without the permission of the former copyright owner." 1
Works published before 1923
Publications created by the U.S. Government
For works created after January 1, 1978: life of the author plus 70 years
For joint works: life of the last surviving author plus 70 years
For anonymous works, pseudonymous works, and works for hire: 95 years from the year of first publication or
120 years from the year of creation, whichever expires first
How long does a copyright last?
The U.S. Copyright Office indicates that "the term of copyright for a particular work depends on several factors, including whether it has been published, and, if so, the date of first publication." 2 Once a copyright expires, the work becomes part of the "Public Domain" and can be used freely.
1 U. S. Copyright Office. (2010). Where is the public domain? Retrieved from http://www.copyright.gov/help/faq/faq-definitions.html#public_domain
2 U. S. Copyright Office. (2010). How long does a copyright last? Retrieved from http://www.copyright.gov/help/faq/faq-duration.html#duration