However, there are rules as to if and when something enters into the Public Domain. A safe rule of thumb: if you do not know if it is in the Public Domain, it is probably protected by copyright!
1. the term of copyright for the work has expired
2. the author failed to satisfy statutory formalities to perfect the copyright
3. the work is a work of the U.S. Government
Most works are considered in the public domain because of old age. This includes any work published in the United States before 1923, those works published before 1964 and whose copyright was not renewed, and a few works that were published without copyright notice (copyright notice was necessary for works published in the United States before March 1, 1989).