Works that are no longer subject to copyright law or that were never protected under copyright law are considered to be in the Public Domain. These works are freely available for public use without the need of a license or permissions.
Ways a work may enter the public domain:
Though copyright protection lasts for an extended period of time, even after the author is deceased, it is temporary. Once the period of copyright protection has expired, all legal restrictions cease. Cornell University provides this chart for copyright terms in the U.S.
Some works do not qualify for copyright protection because they were produced before copyright laws existed, they don't meet the terms of originality, or because they are not protected forms of work. Other works not protected by copyright laws:
Works created by the U.S. Government (with the exception of those created under contract)
Intangible works (ex: improvisational comedy perfromances that have not been written down or recorded)
Ideas, facts, procedures, processes, methods, and systems
Works of common knowledge with no original authorship (ex: calendars, phone books)
Words, names, slogans, short phrases, titles
A creator of a work may choose to forfeit his or her rights by dedicating the work to the public domain. In this case all copyright restrictions are entirely removed voluntarily. This is not the same thing as freely licensing the work, although it is similar.
Questions about whether a work is in the Public Domain? Use this slider tool to help you!
Are modern reprints of a public domain work subject to copyright protection?
No, unless the work contains new material that constitutes a creative change to the original work. For instance, a reprint with new illustrations, a new introduction, or other creative contributions would qualify the reprint as a derivative work.
A reprint of Moby Dick, for instance, would not be subject to renewed copyright protection. A re-issue of Moby Dick containing a new forward by a scholar of American Literature would, however, be protected under copyright law
Are translations of public domain works protected under copyright law?
Yes, a translation is considered a derivative work and is afforded copyright protection.
Public Domain materials are free, right?
Possibly, if a public domain work has been digitized, you may be able to find it for free online. But public domain truly refers to the intellectual property rights, meaning you are free to use the work without any type of permission, even though you may have to pay for it. On the flip side, just because something is free, doesn't mean it isn't protected under copyright law.
What are Orphan Works?
Orphan works are still presumably protected by copyright, they are not commercially available, and the copyright holder cannot be found. Orphan works are rather troubling because no one is benefitting from them, not the copyright holder, not the public, not even the people or organizations who contol access to the materials. Read more about orphan works in this article from The Chronicle of Higher Education, "Out of Fear, Colleges Lock Books and Images away from Scholars."
These books (and more) are possible because the original works are in the Public Domain.