United States copyright laws are complex, and have been changed several times since original implementation. Obtaining permission from the copyright holder for the work you would like to use is often the safest route to ensuring you are not in violation. A great guide on how exactly to do this, including examples of letters for contacting copyright holders, was created by Columbia University.
However, exceptions to the above include Fair Use, Public Domain, and Creative Commons, and in many cases, using copyrighted materials in the classroom likely falls under Fair Use practices. However, before using materials in your courses, it is best practice to check and make sure.
This section of the guide offers a number of resources to help faculty and graduate students determine whether a material can be used.
Jump to Information About:
FAQs (will take you to a different page of this guide)
What is Creative Commons? (will take you to a different guide)
Fair Use (will take you to a different page of this guide)
Everything You Need to Know about Copyright in Education (will take you to a different page of this guide)
Used with permission under CC-BY license by GFCGlobal.
As of January 1, 2019, works created in 1923 or earlier have fallen into the public domain. This article by Glenn Fleishman of the Smithsonian easily explains what this means, and this SCI Google Group post by Scholarly Communications Initiatives Librarian Andrea Wirth provides some additional explanation and further sources on the topic.