Mixed, compatible tunings.
General mountain dulcimer or music discussions
Here's the problem: When you tune to 158, the ti flat note is at the 6th fret. But when you change that tuning to 155, the 6th note of the scale is now at the 1+ position. If you don't have the 1+ fret, you can't play mixolydian songs in the ionian tuning. You will find some mixolydian tunings that only use 5 notes of the scale, but one of those 5 notes will always be the 6th note of the scale. I challenge anyone to find an exception to this. Further, mixolydian songs, when harmonized with chords, use a curious chord built from the seventh note of the scale. In the key of D mixolydian, this odd chord is C major. Modern music theory tells us the key of D contains no C major chord. The ionian scale of D has a C#, so there is no C major there. The ionian chord in this case is C# diminished. It contains C#, E, and G. Modern music theory calls this a DISCHORD, rather than a chord. When the DAA player plays this dischord against the DAD players C major chord, the true meaning of cacophany really shows through. Put this into a true mixolydian song, such as Old Joe Clark. In DAD, we have a true C major chord to play at this point in the song, and it harmonizes perfectly. DAA players need the C natural at the 1+ or 8+ fret to make this C chord. Lacking that, most use A major, or A7. But either of these contains the offending C# note. If either of these is played against the DAD players C major, the C and C# clash. The strongest dissonance in music is to be 1/2 step away from correct. In other words, the C# is more dissonant than D would be in the same situation. And in DAA or DAD, one of the drones is D. That's why the drone doesn't clash with the C major chord. Adding a D to the C chord makes it a C suspended 2. A little jazzy, but not dissonant. The other drone is A. A added to the C suspended 2 gives us C6 susp. 2. Again, jazzy, but not truly dissonant. It doesn't clash. This is why I said yesterday that when playing chords it could cause problems. The drones are D and A in either tuning. No clash at all.
The chords follow the scale, and each mode is it's own scale. Only one note is changed as we go from Ionian to Mixolydian, the C# of the Ionian becomes the C natural of the Mix mode. But the common A chord from the D ionian scale is no longer available. We now have three chords we didn't have in the ionian mode: C major, instead of C# dim Am instead of A major, and F# b5 instead of F#minor. The chord harmonies of this mode use the chords available with this mix scale, though I can't recall seeing the F# b5 used in folk music. A lot of mix mode songs in the key of D use only 2 chords, invariably it is the D and C chords. More often, mix songs will use 3 chords, the D, G, and C chords. The Am is somewhat more rare. A 2 chord Ionian mode song in the key of D will use D and A or A7. 3 chord songs will use D, G, and A/A7. (I am speaking predominantly of folk music, Jazz based songs are the rattle snakes of music; they go where they want and no one tries to stop them!
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Paul
So my music 'theory' tends to be reverse engineered based on what's actually happened 
But with retuning I can use what I do know but in different keys. So different strokes for different folks. All's good. 
