Fair use is a concept baked into US copyright law, and it’s a defense to copyright infringement if certain elements are met. The US Copyright Office says the defense is decided on a case-by-case basis. “The distinction between what is fair use and what is infringement in a particular case will not always be clear or easily defined. There is no specific number of words, lines, or notes that may safely be taken without permission. Acknowledging the source of the copyrighted material does not substitute for obtaining permission,” the US Copyright Office says. There are, however, at least four factors that judges must consider when deciding fair use: the purpose of use, the nature of the copyrighted work, the amount and substantiality of the portion taken, and the effect of the use upon the potential market. — Ars Technica