6 1/2 in ionian mode
General mountain dulcimer or music discussions
@tonyg, if you are tuned Ddd (or DAd for that matter) and play a song in the ionian mode, you should be using the 6-1/2 fret. The 6 fret would be mixolydian.
If you did not have a 6-1/2 fret, you would have to tune your melody string to A to get the ionian mode, and your base or tonic would be the 3rd fret.
The reason is simply that the songs you have chosen employ the major scale or ionian mode. Other songs--most obviously Old Joe Clark--employ the mixolydian mode. Some songs are actually pentatonic, and only involve a 5-note scale. Those can usually be played in either the ionian or the mixolydian modes. But truly modal melodies can only be played in one mode.
In the key of D, the ionian mode makes use of the C# (the 6+ fret on a D string) and the mixolydian mode makes use of the C natural (the 6 fret on a D string).
To play modal music, dulcimer players get accustomed to changing tunings. Or, they add frets. The main reason for the popularity of the 6+ fret is that it lets dulcimer players play in both the ionian and mixolydian modes without changing tunings.
I hope that clarifies more than it muddies.