Call and response is a good term Stephen, as is Colin's question and answer. I think it's kan ha diskan in Brittany, and no doubt there are other terms from other places. I'd noticed this in pipe tunes (e.g. Lasses Boozes Brandy, Sweet as Sugar Candy, Cuddie Claw'd Her) and I'd referred to the instrumental 'responses' as mid-tags and end-tags, but as you point out they are the recurring phrases of songs, and this is borne out in cases where the lyrics survive.
I think it's an important observation of Colin's that the A and C lines (but not the B and D) are varied. I think you'll also find that the A and C in an individual instrumental strain (as opposed to a song lyric) are often the same as each other. If you add the element of harmony alternating with discord against the drones (most often 3 of one and 1 of the other), you have many of the procedures needed to make what we recognise as a traditional pipe tune - which will only be a *good* tune if there is a strong melodic idea present. -- To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
