What you can legally do with it a work under copyright protection and how you may use it is called Fair Use.
Copyright law is written to encourage the growth of knowledge while at the same time protecting those who further the growth of knowledge. You use a book to gain knowledge; an author publishes a book to earn money. You would not purchase a book if you could not use the information in the book, but an author could not publish a book if he could not earn money for his time and effort. "Fair Use" is the concept within the copyright law which weighs the delicate balance between use and protection.
Fair Use is not a set of laws or rules, but a defense to copyright infringement based on the following criteria:
Determination of Fair Use is weighed by looking at how the work is used, how much of the work is used, and probably most important how does the use affect the value and potential sales of the original work. Fair Use frequently comes into question when determining what portion of a book might be quoted or used for reviews, criticism, education, and research. Since this site is about music, we will not delve into details concerning books or other creative works. It is important that you be aware that copyright laws and research methods discussed on this web site may be quite different for works other than music.
Public Domain Research and Fair Use
When doing public domain research, particularly in libraries, be cautious when photocopying music. If the book itself is in the public domain, you may freely copy as much or all of the book as you like. However, there may well be public domain music found in a book which is not public domain. If the book is still under copyright protection, check with the music reference librarian before you photocopy the book.
However, do remember that for music, it is the music notation (the "tangible copy" of the music) that is protected under copyright. If the music is public domain, you can write your own music notation manuscript if there is a problem with your making a photocopy.
Music and Fair Use
We have attempted to do find specific details and examples of Fair Use of music. The rumors that it is OK to use so many notes or so many bars are just not true. There is little doubt that, other than private in-home listening and playing, Fair Use of music is extremely limited. However, we have been able to find almost no discussion of this area outside of legal books clearly meant for attorneys. We have received several conflicting opinions from teachers and college professors about the Fair Use of music in education, especially involving student film and video projects. We highly recommend that you consult an attorney or rights clearance agency before you use any music under copyright protection for anything other than your own personal use.