Authors' rights under copyright law are subject to certain limitations, including the doctrine of “fair use” that was developed through a substantial number of court decisions over the years and has been codified in section 107 of the copyright law. In the U.S., the Fair Use doctrine
allows limited use of copyrighted material without requiring permission from the rights holders under a four-factor balancing test
Whether the use in question helps fulfill the intention of copyright law to stimulate creativity for the enrichment of the general public, or whether it aims to only "supersede the objects" of the original for reasons of personal profit.
In general, the less that is used in relation to the whole, e.g., a few sentences of a text for a book review, the more likely that the sample will be considered fair use.
Courts often consider two kinds of harm to the potential market of the original work. First, courts consider whether the use in question acts as a direct market
substitute for the original work. Second, courts also consider whether potential market harm might exist beyond that of direct substitution, such as in the potential existence of a licensing market.
As noted by the U.S. Copyright Office
, the distinction between fair use and infringement may be unclear and not easily defined. There is no specific number of words, lines, or notes that may safely be taken without permission and acknowledging the source of the copyrighted material does not substitute for obtaining permission.
The 1961 Report of the Register of Copyrights on the General Revision of the U.S. Copyright Law cites examples of activities that courts have regarded as fair use: “quotation of excerpts in a review or criticism for purposes of illustration or comment; quotation of short passages in a scholarly or technical work, for illustration or clarification of the author’s observations; use in a parody of some of the content of the work parodied; summary of an address or article, with brief quotations, in a news report; reproduction by a library of a portion of a work to replace part of a damaged copy; reproduction by a teacher or student of a small part of a work to illustrate a lesson; reproduction of a work in legislative or judicial proceedings or reports; incidental and fortuitous reproduction, in a newsreel or broadcast, of a work located in the scene of an event being reported.”