Copyright is designed to give authors an incentive to create original works by affording them a limited-term monopoly on their intellectual creations.
Copyright is best understood as a suite of five rights:
A work is in the public domain if:
Once a work falls into the public domain, it is no longer copyrightable and anyone may use public domain works without the permission of the copyright holder, who is no longer entitled to the exclusive rights of a copyright holder. These works can be excellent choices for use in teaching and research because there are no permissions required for their use.
Determining whether or not a specific work has fallen into the public domain can be difficult, this Digital Copyright Slider can help. Other sources for public domain material and research on the public domain: