17 U.S.C. § 107 : US Code - Section 107: Limitations on exclusive rights: Fair use

      Notwithstanding the provisions of sections 106 and 106A, the fair
    use of a copyrighted work, including such use by reproduction in
    copies or phonorecords or by any other means specified by that
    section, for purposes such as criticism, comment, news reporting,
    teaching (including multiple copies for classroom use),
    scholarship, or research, is not an infringement of copyright. In
    determining whether the use made of a work in any particular case
    is a fair use the factors to be considered shall include - 
        (1) the purpose and character of the use, including whether
      such use is of a commercial nature or is 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.

    The fact that a work is unpublished shall not itself bar a finding
    of fair use if such finding is made upon consideration of all the
    above factors.