Public Domain is creative work that is not protected by copyright and may be freely used by everyone. Works are considered Public Domain if:
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Work is published before 1923
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Copyright for work has expired
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Work of the U.S. government; federal documents
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Author failed to satisfy statutory formalities to perfect copyright
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Facts, ideas, systems, or methods of operation
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Freeware or unlicensed software
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Published before 1964 and copyright was not renewed. Renewal was a requirement for works published before 1978
There are both copyrighted and not copyrighted materials online. Always assume a work online is copyrighted.