Works in the public domain are not subject to the protection of copyright law. Unless these works are protected in some other way, such as trademark registration, they may be used by anyone for any purpose.

The most frequently used public domain works are classic literature or music. If the public domain status of an item seems uncertain, confirm its availability prior to duplicating or using the material. 

Most of the works likely to be used in the classroom fall into the public domain 70 years after the death of the author, artist, or creator. Prior to the expiration of copyright, permission to use the work is required unless a fair use analysis indicates otherwise.

Works created by the U.S. government are immediately available in the public domain.

Works fall into the public domain for four main reasons:

1. the term of copyright for the work has expired

2. the author failed to satisfy statutory formalities to perfect the copyright

3. the author expressly released the work into the public domain (attribution may be required)

4. the work was created by the U.S. Government

Many works enter the public domain because of old age, and this includes any work published in the United States before 1923. Another large block of works fall in the public domain because they were published before 1964 and copyright was not renewed (Renewal was a requirement for works published before 1978). A smaller group of works fall into the public domain because they were published without copyright notice (copyright notice was necessary for works published in the United States before March 1, 1989).