It's funny; when I'm away from a keyboard, if somebody asked me, I generally wouldn't be able to tell them which thumb I use for pressing the space bar. The obvious approach to finding out would be to do some mock-typing on a table and see which thumb I find myself using, but that never quite seems to work, partly because tables don't feel enough like keyboards and mostly because when I know I'm doing it as a test I feel self-conscious and am never quite confident I'm doing what I would naturally do.

But today I know very clearly which thumb it is, because as of last night I have a nasty bruise under the right side of my left thumbnail, and now it is very obvious to me that that's the bit I normally use for thumping on the space bar. Ow.

(Still, I've only been at work for a quarter of an hour and I've already evolved a typing posture which just about avoids the most painful bit. I suspect this is an advantage of never having formally learned to touch-type; my ad-hoc typing style is naturally adaptable in the face of the unexpected.)