In the United States, a copyright owner can significantly enhance the protection afforded by a basic copyright. This is done by registering the copyright with the U.S. Copyright office. See Copyright Registration and Enforcement . Copyright protection rules are fairly similar worldwide, due to several international copyright treaties, the most important of which is the Berne Convention. This protection must last for at least the life of the author plus 50 years, and must be automatic without the need for the author to take any legal steps to preserve the copyright.
Most works enter the public domain because their copyrights have expired.
To determine whether a work is in the public domain and available for use without the author's permission, you first have to find out when it was published. Works published after 1922, but before 1978 are protected for 95 years from the date of publication. If the work was created, but not published, before 1978, the copyright lasts for the life of the author plus 70 years. However, even if the author died over 70 years ago, the copyright in an unpublished work lasts until December 31, 2002. For works published after 1977, the copyright lasts for the life of the author plus 70 years.