Works that are ineligible for copyright protection or for which copyright has expired are considered to be in the public domain. Such works may be the foundation of new creative works and quoted extensively. They can also be copied and distributed to classes and be included on course web pages without obtaining permission or paying royalties.
In an effort to end what amounts to perpetual protection, Congress allows unpublished works to begin entering the public domain in a gradual fashion. Unpublished works began to enter the public domain on December 31, 2002, as long as the creator had been dead for at least 70 years. At the end of each subsequent year, additional works meeting these criteria will enter the public domain. View a chart showing the current copyright limits.
Keep in mind:
Government documents are works created by United States government employees as part of their official duties; as such, they are not eligible for copyright protection. However, users should be aware that certain distinctions do exist within this framework: works created by quasi-government agencies and those created by contractors for government agencies may enjoy copyright protection.