This article is from the Piano General Topics FAQ, by Isako Hoshino firstname.lastname@example.org with numerous contributions by others.
The "fair use" of a copyrighted work, including reproduction
as copies or recordings for purposes such as criticism,
comment, news reporting, teaching (including multiple copies
for classroom use), scholarship, research, or parody, is not
an infringement of copyright. Fair use is covered in Section
107 of title 17. There is no real definition of fair use,
and in court cases each situation is decided based on its own
facts. However, four yardsticks have come to be used, which
are expressed in section 107 as:
"(1) the purpose and character of the use, including
whether such use is of a commercial nature or is for
non-profit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in
relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for
or value of the copyrighted work."
Most of the applications of the "fair use" concept have to do
with books and articles as used in teaching.
...in a nutshell...
Fair use includes:
a) reviews/criticism (including parody),
b) scholarly use (photocopying one section only--
not a complete perform-able portion such as a movement
or an aria; and for study purposes only, not for
"using"--such as photocopying a copy for each student
to use for a form & analysis exercise or test).
c) Copying a page to avoid a page turn is generally
considered fair use.
Fair use does not include:
a) copying to avoid buying the book
b) because it's out of print
c) because there's not enough time to order and receive
d) because you can't find who holds the copyright
e) because you need one for your duet partner.