The duration of copyright protection recently changed as a result of the Sonny Bono Copyright Term Extension Act of 1998. The easiest rule to state is that copyrights have expired on all United States works registered or published prior to 1923. As a result, all such works have entered into the public domain.
After 1923, it is more complicated to determine when a copyright will expire. Like the old provisions, the duration of copyright protection under these new provisions depends upon when the work was created and first published.