Transposing instruments · Transposition · Music theory
Some instruments sound at a different pitch from what is written. A B♭ clarinet reading a C produces a B♭: everything sounds a major 2nd lower than…
Transposing instruments
Some instruments sound at a different pitch from what is written. A B♭ clarinet reading a C produces a B♭: everything sounds a major 2nd lower than written. That is why its part is written a major 2nd higher than the real music, with the matching key signature.
The name of the instrument says it all: it is the note that sounds when C is written.
Why transposing instruments exist
There are very practical historical reasons. Old horns and trumpets, without valves, could only play the harmonics of a tube; to play in another key a length of tube, the crook, was changed, and the music was always written as if in C. The instrument changed and the player’s positions did not.
Instrument families work similarly: saxophones or clarinets of different sizes share fingerings, so someone who plays one can read the others without learning a new reading. Other instruments are written one or two octaves away just to avoid ledger lines.