This means that an unauthorized use of copyrighted material is excusable if it falls under the principle of fair use. Although the law does provide guidelines for making this assessment, determining fair use is not always easy since it is a grey area of the law. Commentary and Criticism. If you are commenting upon or critiquing a copyrighted work—for instance, writing a book review—fair use principles allow you to reproduce some of the work to achieve your purposes. 1. Most fair use analysis falls into two categories: (1) commentary and criticism, or (2) parody. Fair use is a defense to copyright infringement. In the United States, fair use is determined by a judge, who analyzes how each of the four factors of fair use applies to a specific case.