This has been an interesting and generally well-informed discussion. Can I throw in my fourpennyworth.

There are a number of tunes in minor modes (let's take Em) as an example, where a change to a chord of C can be very effective (or F if you are in Am). For example, in the Swallow Tail jig, we used to play the sequence Am G Am Em, but now play Am G F Em. This gives a nice run down on the bass and sounds good, even though it is a slight dischord. Another example might be the morris version of Greensleeves, where you can play the first 4 chords as Am G F Em, and it also works in the Bear Dance B music, etc.

The other point that I have noticed recently is that some bands/musicians will end a musical phrase not on the chord of the Tonic but on the chord of the Dominant. In other words, if you are in G (think of, say, the Oyster Girl). end the tune on the note of G but play a D chord against it. It's a dischord of course, but it says, 'we haven't finished with this tune yet, there's more to come.' The Askew Sisters are particularly good at this technique and you can hear it in other groups as well (Faustus for example). You can't really do it in a session as it would produce a horrible clash, but in a group it can work.

Tradsinger