A work in the public domain is not protected by copyright. You are free to copy the entire work. In the United States, works published before 1923 are in the public domain. Authors may also dedicate their work to the public domain.
The basic intent of copyright law is to promote the progress of science and other useful arts (promote knowledge). This is done by protecting works for a period of time to encourage artists and authors to produce more materials. However, once the protection expires, anyone can use the work for further expression.
Once material enters the public domain, their use fosters new creation and expression resulting in new works created from existing public domain material. The public benefits from both the newly created materials and more readily available low cost editions of existing public domain material. Works without copyright protection are included in the public domain and are available for everyone to use freely.
The majority of documents published by the federal government are in the public domain. There are some exceptions. The federal government out sources some of its research and publications to private publishers. Those works may be copyrighted. Check the specific document.
Also many state and county publications may be copyrighted; they are not necessarily in the public domain.
The public domain consists of works that are either ineligible for copyright protection or when a work's copyright protection has expired.
The public domain also includes:
As a general rule, most works enter the public domain because of old age. This includes any work published in the United States before 1923. Another large block of works are 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 fell 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).
It is difficult to determine if a work published after 1923 is in the public domain. The copyright for some works published prior to the 1976 law may not have been renewed. The Copyright Office maintains a database of copyright registrations back to 1978. The Copyright Office also publishes the circular "How to Investigate the Copyright Status of a Work." For works published prior to 1978, the Copyright Office can do a manual search of its records for a fee. See Ready Reference on this page for additional options when search for copyright renewal information.
A work can also be an "orphan work," meaning that it may still be under copyright, yet no rights holder can be found.
Copyright owners can modify or surrender their copyright their work by dedicating it to the public domain or granting licenses to others for use. Copyright owners can control levels of use by designating one of 6 layers of licensing through Creative Commons (http://creativecommons.org/).