In the United States, copyright law has its foundation in the US Constitution. Article 1, Section 8 states:
The Congress shall have the power to [...] promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writing and discoveries.
The 1976 Copyright Act (17 U.S.C. § 101 et seq.) established copyright law in the US as we know it today. From 1976 to October 2007, there have been fifty-nine amendments to the law. (U.S. Copyright Office: Copyright Law)
UNC Copyright Policy
Except as allowed by law, it is a violation of [UNC Copyright Policy] and law for University faculty, staff, or students to reproduce, distribute, display publicly, perform, digitally transmit (in the case of sound recordings), or prepare derivative works based upon a copyrighted work without permission of the copyright owner.
Copyright law covers "original works of authorship fixed in any tangible medium of expression from which they can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device" (Copyright Act, 17 U.S.C. § 102).
Types of original works include literature, music, drama, computer programs, visual art, and many other forms of expression.
Copyright protection does not extend to ideas, facts, or procedures.
By their definition in the U.S. Constitution, copyright terms are limited. How long a work's copyright term lasts depends on when the work was published. Here are some general rules for textual works published in the U.S.
works published in the U.S. before 1923: copyright has expired
works published in the U.S. between 1923 and 1977 with copyright notice: copyright lasts for 95 years after date of publication
works created between 1923 and 1977 but not published: copyright lasts for 70 years after the life of the author
works created or published in the U.S. after 1977: copyright lasts for 70 years after the life of the author
works published anonymously in the U.S. or works for hire in the U.S. after 1977: copyright lasts for 95-120 years
Copyright also does not cover works in the public domain. These include:
works whose copyright has expired
works published in the U.S. before 1989 without copyright notice
works created by US federal government employees as part of their employment (However, some works might be done by an independent contractor or include copyrighted material, so always check the copyright information carefully.)
Works in the public domain can be used freely.
Examples of Protected and Unprotected Works
The following works are all protected by copyright law:
a poster created for a conference in 2006
a presentation at a conference last month
an image created for a textbook in 1990
a paper written for a Fall 2005 class and never published
However, the following works are not protected by copyright law:
an article published in the Journal of the American Medical Association in 1902 (too old)
an article in a CDC publication that was published in 1996 (government document)