The Major Scale
The tuning of the diatonic harmonica is based on one major scale defined by the key the harmonica is in. In the case of the C diatonic harmonica, all the notes are based on the C-major scale. Although the blow notes on the C harp make up three C-major triads (the notes C, E, and G repeated three times with one more C on top), it's interesting to note that, because of the way the draw notes are laid out, each of the three octaves create different scales. Playing the notes consecutively beginning with hole 1 blow-draw, the first octave is C, D, E, G, G, B, C; the second octave is C, D, E, F, G, A, C; and the third octave is C, B, E, D, G, F, C, with A as the final draw note.