In Hannigan and Ledsam’s Low Whistle Book (and other places, I think) they recommend putting the thumbs behind the second finger of each hand.
I find this uncomfortable and put my thumbs behind the top finger of each hand, or maybe a bit below.
Is there a good reason for this recommendation? Something to do with pipers’ grip? Should I try to become comfortable with it?
Since my thumbs are ridiculously long, my left thumb is actually above the first finger and my right thumb is just behind the first finger. I also broke my left thumb quite badly years ago, so I have to find a spot that generates the least stress and still allows me to control the whistle. I think is comes down to personal preference.
I don’t think it matters much.
Theory, seems to me, would be behind having each thumb more or less centered behind each hand (if the purpose is to act as a balance or support).
My first instrument was Highland pipes, then uilleann pipes, then flute, then whistle and I’ve always placed my lower-hand thumb between middle and ring fingers. (This has had an unintended advantage when I play a chanter or whistle with a hole on the back for F natural, the hole is right where my thumb is anyhow.)
On pipes my upper-hand thumb was placed above the hand altogether, above the index finger, because pipes have a thumb-hole placed higher than the top finger-hole . On flute (traditional wood flute with keys) the upper-hand thumb has to be lower, placed to actuate the Bb key. These habits get carried over when pipers and fluters play whistle, probably.
BTW you can often infer whether a whistle-players comes from flute (Boehm or Old System), pipes, or recorder by watching their thumb. Oftentimes recorder players move their thumb on and off the back of the whistle out of habit.
Some whistles have a C natural thumb-hole in analogy with Boehm flute so of course the thumb has to be placed between the index and middle finger to cover it.