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Works fall under the public domain through:
Table for Determining Public Domain Status
Works published in the U.S. before 1928 | In the public domain |
Works published in the U.S. after 1928 through 1963 | Initial term of 28 years. If not renewed during the 28th year, the work falls into the public domain |
Works published in the U.S. after 1927 but before March 1, 1989 | Generally, if a work is published without copyright notice under the authorization of the copyright owner and the law does not provide an exception for the omission, the work is in the public domain |
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Copyleft is a concept that developed in the computer software community and led to the GNU public license.
Copyleft is a general method for making a program (or other work) free, and requiring all modified and extended versions of the program to be free as well.