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 criticism, comment, news reporting, teaching (including multiple copies for classroom use [however there is a limit to this use; read this scenario], scholarship, or research, is not an infringement of copyright" (Circular 21. Reproduction of Copyrighted Works by Educators and Librarians, C. Fair Use,1. Text of Section 107, § 107 - Limitations on exclusive rights: Fair use).
For a more complete understanding, read Journal Article for Classroom Use, SCENARIO 1, then scroll down to Out-of-Print-Book, SCENARIO 10 and SCENARIO 11], scholarship, or research, is not an infringement of copyright" (quoted from: Section 107. Limitations on exclusive rights: Fair use).
There are no legal rules permitting the use of a specific number of words, a certain number of musical notes, or percentage of a work. Whether a particular use qualifies as fair use depends on all the circumstances (something called the four fair use factors). See
Also note that the so-called "rule of thumb" where it's always OK to copy 10% of a work or one chapter, whichever is smaller, is not true!
Read through each of these 10 short Scenarios (scenarios 1, 10 and 11 were already referred to above) for a more complete understanding.
Copyright and fair use can be complicated. My strong recommendation is: "It is better to be safe than sorry" and do two things when in doubt if what you are doing is fair use or requires :
1) read this brief, well-cited page from Columbia University Libraries which includes the "famous" Fair Use Checklist [PDF] which are the Four Fair Use Factors.
2) Do fill out the checklist and keep a copy of those results, just in case the intellectual property owner asks "did your use really fall under fair use?"
Here is an example of how one APU faculty member (yours truly) filled out the check list: Fair Use Checklist - an Example