SqueezeMe - I did mention the drones in my first post, but, to elaborate a bit:

I remember Peter showing me one (or possibly even two - it was a long time ago) button in the middle of the LH third (top) row, which was tuned to a drone and had a small metal plate that could be swivelled over to hold it down for the duration of a song. A drone is more usually fitted under the left thumb, but holding that down for several minutes is hard work.

Peter's drone was simply that of the instrument's home key - a C on a C instrument, etc. The unusual chording came because he would add harmonies to it, usually but not always from the I, IV, V chord palette. The more unusual chording came about when he would sound a V chord against the drone so, in the key of C, you would be hearing G, D and C notes together, making an edgy, clashing sound (can Phil tell us what that's called?).

Some of the strangest sounding accompaniments were on songs that already had unusual melodies. 'Streets of Derry' is played against a simple, home key drone, but the nature of the melody is such that a lot of it ends up on the pull, with plenty of opportunity for clashes against the drone. Incidentally, listening to this track carefully, I'm struck by the number of little ornaments Peter puts into the accompaniment.

On 'Housecarpenter' (both these tracks are on 'Both Sides Then'), he's not only playing a drone, but he's accompanying a F mixolydian melody in the key of Bb, so none of the chords are the ones you'd normally expect. With the Bb drone added, it sounds incredibly eerie, fittingly for a ghostly story. Martin Carthy used to do the same kind of thing on guitar - he called it 'cross-tuning' - the two were good pals so perhaps they compared notes.