What is the "Fair Use" Doctrine in Copyright Law?

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U.S. law permits limited “fair use” of copyrighted material without consent of the copyrighter. Fair use is an affirmative defense to a claim of copyright infringement. Courts consider four factors when determining whether an alleged copyright infringement falls under fair use: (1) the purpose and character of the use, including whether such use is of a commercial nature or for nonprofit educational purposes, (2) the nature of the copyrighted work, (3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole, and (4) the effect of the use upon the potential market for or value of the copyrighted work. Parodies are a well-established and accepted form of fair use.