The public domain is not a place.
A work of authorship is in the “public domain” if it is no longer under copyright protection or if it
failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner.
These include items that are not eligible for copyright protection such as ideas, facts, concepts and principles. The public domain also includes works for which the copyright protection has expired, federal government documents, and works that a copyright owner grants to the public domain.
For all U. S. works registered or published prior to 1923 the copyrights have expired. As a result, all
such works have entered into the public domain.