There is no set number of words you can use under what’s called the fair use doctrine. This doctrine permits use of copyrighted materials without the owner’s permission for certain purposes listed in the Copyright Act, such as criticism, comment, news reporting, teaching, scholarship, or research. While technically infringing on the copyright owner’s rights, these uses are considered permissible; and such fair use can be used as a defense against a claim of copyright infringement. Note that it is not sufficient simply to acknowledge the source of the copyrighted material.
The Copyright Act provides four factors to determine, on a case-by-case basis, whether fair use or infringement exists.
1. Character or purpose of use
2. Nature of the copyrighted work
3. Amount and substantiality of the copied work
4. Effect on the potential market
The concept of fair use should not be confused with the concept of public domain. When a work is in the public domain it has no copyright protection. Therefore, the public —rather than a particular individual or entity—owns the work.
A work might be in the public domain for one of four reasons:
1. The term of copyright protection has expired
2. The owner failed to fulfill a requirement and lost copyright protection
3. The work was created by the U.S. Government
4. The owner dedicated the work to the public domain
As a rule of thumb, registered works created before 1923 are now in the public domain.
Click here for more information about copyright, fair use and public domain.