Determining the Copyright Status of a Work
It can be difficult and time-consuming to determine the copyright status of a work, given that copyright laws have changed over time. Works on which the copyright term has expired and works which were never covered by copyright (such as US government documents) are said to be in the public domain.
Two excellent resources to assess whether a work has passed into the public domain:
- Copyright Term and the Public Domain in the United States , a chart created by Peter Hirtle at Cornell
- When US Works Pass Into the Public Domain, by Lolly Gasaway
Some other useful resources: