A work in the public domain is either ineligible for copyright protection or the copyright has expired. Anyone can use and copy any work that is in the public domain without the necessity of obtaining permission or the need to pay royalties. There are certain categories of works that are considered in the public domain:
- U.S. government works
- Ideas and facts
- Scientific principles, theorems, mathematical formulae, laws of nature
- Scientific and other research methodologies, statistical techniques and educational processes
- Laws, regulations, judicial opinions, government documents and legislative reports
- Words, names, numbers, symbols, signs, rules of grammar and diction, and punctuation
- Works with expired copyrights
There are also a few general rules with regard to works that once were protected by copyright that have entered the public domain:
- Any work published in the United States prior to 1923 is automatically considered without question in the public domain,
- Works published in the United States between 1923 and March 1, 1989, are in the public domain based upon whether the certain statutory formalities were observed, such as giving notice of copyright on the work or renewing the copyright per statutory deadlines.
- Works published between 1923 and 1978 without a notice, are considered in the public domain. Works published between 1923 and 1978 with notice are protected for 95 years from the date of publication.
- Works published in the United States between 1978 and March 1, 1989 without a notice and registration, are considered in the public domain. (Works published during this period with a notice, but not a registration, are protected for 70 years from the death of the author.)
- Works published after March 1, 1989 are protected for 70 years from the date the author dies unless it was a work for hire for corporate authorship where the copyright term is 95 years from publication or 120 years from creation.