Series: Belwin Course for Strings Category: String Orchestra Method/Supplement Format: Book Instrument: Cello Instrumentation: Cello The Duets for Strings, Book 1 may be started when the pupil has reached page 9 of Book 1 of the String Builder. They may, however, be used in conjunction with Book 1 of any standard string class method. Published in three levels for violin, viola, cello, and bass.