I've been playing guitar and bass for about two years now, and recently picked up the trumpet. After comparing the notes on trumpet and guitar, I realized that trumpet is a step lower than guitar. So "C" on the guitar and piano really isn't universally a "C". As I've thought that notes were completely solid and never changed in this way for the past few years, the concept of these "transposing instruments" really caught me of guard. What is the purpose of transposing instruments and why is the music system even set up this way? I'm just confused, could someone shed a little light on this for me?