Currently, copyright is automatic when an original work is created in a fixed form in the United States. It lasts 70 years after the death of an author if created after 1978.
Copyright law, in its earliest form from the 18th century, was intended to cover only books. Through time, other items were included for copyright protection:
Copyright protection falls under Title 17 of the U.S. Code and covers "original works of authorship."
So what makes a work original?
Facts and Ideas
Legislative or Administrative or Statues
Public Domain
"The public domain is generally defined as consisting of works that are either ineligible for copyright protection or with expired copyrights. No permission whatsoever is needed to copy or use public domain works. Public domain works and information represent some of the most critical information that faculty members and students rely upon. Public domain works can serve as the foundation for new creative works and can be quoted extensively. They can also be copied and distributed to classes or digitized and placed on course Web pages without permission or paying royalties." UCCopyright
As copyright expires, a work moves into the public domain as follows:
A work is subject to laws of the country of origin, any international laws, and the Berne Convention. Availability in the public domain can be limited by international rules.