Copyright

Things in the Public Domain

Works enter the public domain in several different ways. 

• Generally speaking, works published more than 95 years ago, or unpublished works created more than 120 years ago, are likely in the public domain. This means that as of 2020, works published before 1925 and many unpublished works created before 1900 are possible in the public domain. 
• Once upon a time, a work had to comply with certain formalities in order to acquire copyright. For example, works were required to bear notice and be renewed. Not all works followed these formalities. When copyright holders did not follow these formalities, their works' copyright expired, and the works entered into the public domain.  
• Copyright holders can choose to waive their copyright. For example, you may dedicate your work to the public domain using the CC-0 dedication. 

You can reference the public domain chart created by Cornell to see if a work has already entered into the public domain.

The First Folio of Shakespeare from 1623, for example, is in the public domain. In contrast, a newer edition, with newly added annotation or comments, could still be subject to copyright, but only for parts recently added. Original materials created by Shakespeare cannot be removed from the public domain.

Public Domain and Plagiarism

When you find something in the public domain, you can use it however you'd like without having to worry about copyright law. Plagiarism is another matter: even if something is in the public domain, you should still provide proper citation when you use the work. You are not bound by copyright law to give attribution; you are bound by academic integrity to give attribution when you use someone else's work. 

"Copyright Basics" by Yuanxiao Xu, University of Michigan - Ann Arbor is licensed under CC BY 4.0