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Read Can Arcadia University Use this Image for This Case Study? to see an analysis that uses the balance scale analogy explicitly.
Public Domain Slider
This digital version of an analog tool can help you understand if a work has entered the public domain (in the U.S.) because its copyright protection has expired. If it has, you don’t need to seek copyright permissions and can use it freely. This slider comes from the Copyright Advisory Network of the American Library Association.
Public Domain Chart
For internationally copyrighted works, use this chart from Cornell University’s Copyright Information Center. It also covers works copyrighted in the U.S. as well as sound recordings and architectural works.
Works whose copyrights have not been renewed could now be in the public domain, but how do you know if they’ve been renewed? Take a look at this webpage to help figure that out. It’s from the Online Books Page of the University of Pennsylvania.