These are called "transposing instruments" and the reason for their keys has to do with their ranges, as well as their ability to allow the musician to switch instruments without having to change fingerings. The clarinet originally was in the key of C until the richer sounded Bb clarinet was developed. http://simple.wikipedia.org/wiki/Transposing_instrument Some info on instrument ranges - http://en.wikipedia.org/wiki/Range_(music) Musical notation written for transposing instruments shows the relative pitches, rather than the exact pitches, produced. Writing in this manner is a historical convention that often allows players to switch from a given instrument to a related one without relearning fingerings and other techniques (as from Eâ™­ clarinet to Bâ™­ clarinet or from English horn to oboe). The instrument name, as in the case of the Bâ™­ clarinet, often indicates the pitch resulting when the player sounds the note written as C. If the actual pitch is lower (or higher) than the notated pitch, the written music must be adjusted upward (or downward) by the same amount. Thus, in the key of C major the music for the Bâ™­ clarinet must be written in the key of D major. Although most transposing instruments belong to the woodwind and brass families, transposing keyboard instruments have also been built. The piccolo, contrabassoon, and other instruments whose parts are written an octave above or below the actual pitch (as c′ above c) are not considered transposing instruments. http://www.britannica.com/EBchecked/topic/603166/transposing-musical-instrument