A transposing instrument is one that is not notated at its true pitch but (mechanically and without any effort on the player's part) produces the effect of that pitch. Take the clarinet as an example. It is found best to make it in two sizes, of which the normal keys are respectively B flat and A (the object of this is to reduce the difficulty of playing in the flat and sharp keys, respectively, by reducing the number of flats or sharps with which the player has to cope). Taking the B flat instrument as our example, that key is to its player the 'natural key' (as C is to the pianist): the player faced with music in (say) the key of E flat finds his music written in the key of F, i.e. he has two flats less to consider. Similarly with the A instrument, a piece written in the key of B is notated in the key of D, i.e. he has three sharps less to consider. Thus music for the B flat clarinet is notated a tone higher than it is to sound and music for the A clarinet a minor third higher than it is to sound.necessary transposition mentally. The abolition of the out-of-date method
Many players today, with improved mechanism and developed technique, use the B flat instrument for all keys, making the necessary transposition mentally. The abolition of the out-of-date method of notation would save unnecessary mental effort in conductors and other readers of scores, in which there may be as many as 10 transposing instruments in use at any moment, and these requiring transposition by a number of different intervals; this effort, however, becomes, with practice, subconscious and does not seem unduly to worry practiced musicians.
The transposing instruments are as follows: (a) bass flute; (b) cor anglais, oboe d'amore, oboe in E flat, heckelphone, sarrusophone; (c) clarinet in B flat and A, bass clarinet, high clarinet in E flat and D, alto clarinet in E flat and F, basset horn, pedal clarinet; (d) saxophones; (e) cornets; (f) French horns; (g) trumpets; (h) saxhorns; (i) kettledrums (up to Mozart's period, but excluding Handel).
The brass band is a law to itself in the matter of transposition.
The following instruments, which are notated an octave higher or lower than their actual sounds, are not looked on as transposing instruments: piccolo, double bassoon, and string double-bass.