Depends on the clarinet. You can get C models which will play the piano line as written. B flat clarinet needs to be transposed up a tone; playing C sounds B flat. Used where the key signature has flats. Or there is the A clarinet which needs to be transposed up 3 semitones and is uesd where the key signature contains sharps. There are at least two other clarinets around but the B flat and A are the commonest.