The U.S. Copyright Office is actually a department of the Library of Congress, and was created by Congress in 1897. The U.S. law has been revised several times, including recently to address digital technologies.
Copyright law within the United States (Titles 17, Sections 101-111 of the United States Code) protects "original works of authorship fixed in any tangible medium of expression, now known or later developed, from which they can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device."
Find more information at Copyright.gov.
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