Copyright, public domain, derivative works in general
The copyright is a legal framework that provides an author the right to control how his/her personal work is used, including the exclusive right to reproduce, distribute, adapt, display and perform it. A copyright can protect musical works, as well as sa oftware or a database.
At the contrary, the public domain includes all works who are not protected by a copyright, for different reasons – the work was published before there was a copyright law, the copyright expired or was lost.
The legal status of derivative (= adapted) versions of public sheet music is generally a source of questionment for orchestra and choir directors because sometimes original versions of these derivative works can be difficult to find and in that case it is hard to say if the original work and/or some derivative work is/are or not under copyright.
Actually, to be protected by copyright, a new arrangement of pre-existing work must contain more than some ‘cocktail pianist variations. Something of substance must be added making the piece to some extent a new work with the old song embedded in it’ (Kraslovsky & Shemel, This Business of Music, 1995).
Sheet music and the public domain, what to do to make perform a derivative work in a legal framework
The type of questions that orchestra and choir directors plus music concert organizers must resolve before some performance is:
When was the sheet music written? In which country?
Is the sheet music a derivative work? Is it an arrangement?
Is the sheet music a collective work?
Does the sheet music include public domain elements? (ideas, melodies, titles, musical forms, etc)
For printing, as derivative works are written arrangements of an original piece of music created by a specific composer, they cannot be printed and sold as sheet music by other persons without a licence from the composer (who may charge a high fee for it or refuse to grant it). So it is necessary to ask for permission and pay a fee if the original work is not in the public domain.
Sheet music arrangements and derivative works, the help of music software
I must indicate that composers who want to create a derivative work can find a great support by using Pizzicato music composition and notation software (http://www.arpegemusic.com) because it includes a lot of intuitive features which will help them to develop their creativity around specific pieces of music.