A pianist, a cellist, a trombonist, and a autist all see a C written in their parts. They may play the C in different octaves, but they will all play a note that the others recognize as a C. This may seem obvious, but a clarinetist who sees a C on the page will play a note that does not sound like a C to the other players.
This is because the clarinet is a transposing instrument. The music for transposing instruments is not written or read at concert pitch. The clarinetist, for example, seeing a C on the page, will play a note that sounds like a Bb. The clarinet is therefore called a Bb instrument. A French horn player, seeing a C on his “horn in F” or “F horn” part, will play a note that sounds like an F. Obviously, not just the C but all the notes are different. For a Bb instrument, for example, not just the C sounds a whole step lower, but every note sounds a whole step lower than written. In order to be read correctly by most players, music for transposing instruments must be properly transposed. ….