Section 30(1) of the UK Copyright, Designs and Patents Act 1988 allows “fair dealing” with a copyright work for the purpose of criticism or review, provided that it is accompanied by a sufficient acknowledgement. Under the fair use doctrine of the U.S. copyright statute, it is permissible to use limited portions of a work including quotes, for purposes such as commentary, criticism, news reporting, and scholarly reports. There are no legal rules permitting the use of a specific number of words, a certain number of musical notes, or percentages of a work. Whether a particular use qualifies as fair use depends on all the circumstances. See Circular 21 and FL 102. (From the US Copyright Office FAQ.)