transposing instrument, a musical instrument whose part in a score is written at a different pitch than that actually sounded. Such an instrument is usually referred to by the keynote of its natural scale—the clarinet in A, for example—in which case A is sounded when the tone C appears in the musical notation. Since A is a minor third below C, the part for this instrument must be written a minor third higher than it is to sound. Transposing instruments were necessary in the 17th and 18th cent. when the natural brasses and the clarinets could be played easily in only a few keys; they were therefore built in specific keys. Although improved construction in the 19th cent. obviated this necessity, all clarinets, the English horn, oboe, French horn, trumpet, alto flute, cornet, and most saxophones are transposing instruments. Parts for the piccolo, double bass, and contrabassoon are written an octave below or above actual pitch to avoid ledger lines, but this is not, strictly speaking, transposition.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.