These are instruments for which the music is written in a different key than actually sounding. Transposing instruments were first used for wind instruments before valves were invented and only natural overtones could be produced. During this period a whole set of different sizes of the same kind of instruments had to be used so as to make it possible to produce all the notes. The player had to change instruments every time he was to produce notes which could be produced only on a certain instrument. The real problem was to remember what instrument he had at a certain moment so as to produce the desired notes. To facilitate the problem the composer has to write the music as if the player had the same instrument although he changed it.
A clarinet in B flat,for instance,can easily produce a B flat major scale when the player uses the simple fingering of C major. The composer writes the clarinet part a major second higher than sounded with the player using the fingering of a C instrument.
All the transposing instruments are relatively sounded in the interval above or below the note C. An instrument in E flat, for instance will sound a major third higher than written. some of the instruments are transposed in octaves. The double bass sounds an octave lower than written while the piccolo sounds an octave higher than written. This way ledger lines are avoided.
