I think that the AABB may have something to do with dances and also ease when musicians playing together. With AABB everyone knows where the tune is going (if you pay attention
). If a tune steps outside of this pattern then you have to be on your toes!
If I'm playing a tune like say Angeline the Baker and singing as well then I may well step outside of the AABB patern as the fancy takes me. But if anyone is playing along with me then the A to B changes need some anouncement of some kind if they stray from AABB.
I've been listening to Tommy Jarrell and Fred Cockerham recently and they don't change strictly on AABB, particularly if there is singing involved. I found a nice article on the pair and here is what it says about changes (Charlie is Charlie Jarrell, Tommy's uncle):Tommy and Charlie had a special form of communication when they played together. The common way of playing fiddle tunes is to play the high part twice, followed by the low part twice. Tommy and Charlie would sit close, facing each other. When one got tired of whichever part they were on, he'd give the other a push with his leg. Tommy and Fred continued this tradition, however it was simplified to a raise of the fiddle when Tommy wanted to change parts.
I've listened to quite a lot of old recordings recently and AABB is the most prevelant form I've heard for fiddle tunes, particularly in bands. But for solo players or if singing or hollerin' is involved then this pattern may change.
Mind you - A to B changes are simple to follow compared to those players who randomly throw in a 9th or 17th measure on the turnaround