Hi, Joe. When the melody string is tuned to "d" and the melody string is tuned to "A" and the bass string is tuned to "D," you have the dulcimer tuned to the key of D. This key has two sharps in the scale, A# and F#. On a diatonic fret board those sharps will occur at the proper place in the scale. If the book was written for the CGG, there would be no sharps or flats because then you are tuned to the key of C. A diatonic fret board is defined by the spacing of the frets: long, long, short, long, long, long, short. When the 6 1/2 fret is added you are beginning to move toward a chromatic scale which is all short giving a full twelve not scale while the diatonic is only 7 notes. I think part of the confusing comes from trying to explain that the dulcimer is a simple instrument to play. I hope this helps.
Ken
"the dulcimer sings a sweet song."