545 mm - that's about 21 2/2 inches.

"Amen to that. I would add that, without instrumental accompaniment, group singing tends to be out of TIME even more noticeably and seriously than to be out of TUNE. People can't wait quite long enough between verses unless some instrument, any instrument, fills in the beats between verses."

I can think of any number of folk venues where that just isn't true, and where the singing is all; the better for having no instruments. And I've even known church congregatins who have learnt how to sing without instruments.

"Tends" is a dodgy word - it can mean anything from "almost invariably" down to "quite often".