"If a book, song, movie, or artwork is in the public domain, then it is not protected by intellectual property laws (copyright, trademark, or patent laws) - which means it's free for you to use without permission."
"The words 'public domain' mean creative works that for one reason or another are not protected by copyright law and are ordinarily free for all to use." Another definition of public domain comes from Richard Stim, "The term 'public domain' refers to creative materials that are not protected by intellectual property laws such as copyright, trademark, or patent laws. The public owns these works, not an individual author or artist. Anyone can use a public domain work without obtaining permission, but no one can ever own it."
Stim, Richard. (2016). Getting Permission: How to License & Clear Copyrighted Materials Online & Off. 6th edition. Berkeley, CA: NOLO. Retrieved from: https://bit.ly/2keMSLA
Fishman, Stephen. (2017). The Public Domain: How to Find & Use Copyright-Free Writings, Music, Art & More. 8th edition. [Place of publication not identified]: NOLO. Retrieved from: http://search.ebscohost.com/login.aspx?direct=true&db=nlebk&AN=1477894&site=ehost-live
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