Basically, the diatonic harmonica includes a three-octave extension but doesn't permit you to play all of the notes that form them. The note layout follows a diatonic progression (major scale). With a chromatic harmonica, it is possible to play all the notices like a piano. The technique to perform these two kinds of harmonicas differs, and the sound is quite different too. On the chromatic harmonica, the blues is nearly always played in the 3rd position, and the fact you could play all the notes makes the chromatic harmonica ideal for jazz and classical music.