One of the rights accorded to the owner of copyright is the right to reproduce or to authorize others to reproduce the work in copies. This right is subject to certain limitations found in sections 107 through 118 of the Copyright Act (title 17, U. S. Code). One of the more important limitations is the doctrine of "fair use."
Section 107 contains a list of the various purposes for which the reproduction of a particular work may be considered "fair," such as criticism, comment, news reporting, teaching, scholarship, and research. Section 107 also sets out four factors to be considered in determining whether or not a particular use is fair:
the purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes;
the nature of the copyrighted work;
amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
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. 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."
Copyright protects the particular way an author has expressed himself; it does not extend to any ideas, systems, or factual information conveyed in the work.
The safest course is always to get permission from the copyright owner before using copyrighted material.
When it is impracticable to obtain permission, use of copyrighted material should be avoided unless the doctrine of "fair use" would clearly apply to the situation.