Fair use is a legal exemption to the exclusive rights of copyright holders. It is determined on a case-by-case basis and is based on a consideration of the following four factors:
- The purpose and character of the use (including whether it is transformative, commercial, non-profit, or educational)
- The nature of the copyrighted work
- The amount and substantiality of the portion to be used
- The effect upon the potential market for the copyrighted work
Because intention is a part of the consideration, only the user can make the initial assessment of whether their use is fair. Cornell University Library provides this checklist to help you in your determination: http://copyright.cornell.edu/policies/docs/Fair_Use_Checklist.pdf. We strongly recommend that you complete and retain this form for all Fair Use considerations you make.
Fair use rules do not state a concrete maximum of usable material (not a number of words, or a length of time, or a percent of total). Fair use also does not mean that Cornell users can use/distribute any copyright material they want simply because Cornell is an educational non-profit. We are bound by copyright law too!
There is a lot of subtlety in Fair Use interpretations, for more information see the Copyright Office’s Information on Fair Use, and Case Index.