Ouch, I'm 26 & that's exactly where I tend to hold most pipes. But I'm gonna guess he'd had a pipe in his mouth 8+ hours a day since he was old enough to milk the cows & such.
Honestly, ever since I started following this thread I have been more & more conscious of how often/hard I clench. I can get over some extra wear on the chompers when I'm 80, but I'd rather not be snaggle toothed with a stiff jaw by the time I'm 50. You know you multitask/clench too much when you've smacked a pipe into a door frame & the pipe didn't budge. That rung my bell a bit.
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This got me thinking about exactly what I do when I clench. It's a little more complex than just jamming the thing in my mouth and biting down. There are three parts to my "clench": the lip of the stem tip is hooked under an upper incisor, the lower lip supports the weight and a lower incisor holds it in place under the stem lip to stop it falling out. Because of the angle the pipe sits at I don't need to thrust my jaw forward (no doubt thanks to some expensive othodonstistry when I was a teenager) and no real pressure need be applied. The weight of the pipe holds it in place. I little pressure stops the pipe moving about when I turn my head.
OK I've now devoted far too much time to analsying this.