The head, or knob, is often ornately carved with faces or shapes that have symbolic meaning.

In the 1950s they tried their best to muffle this sound with mercury switches and silent knob controls.

The waiter, you know, came out with the white truffle knob and his shaver, and he shaved it onto my pasta and he said, you know, Would Signore like the truffles?

And what we re doing is we re using humor as a sort of neuroscientific probe by switching humor on and off, by turning the knob on a joke now it s not funny ... oh, now it s funnier ...

But then I knew from experience and looking around in offices where there were chairs with headrests that nobody would ever bother to reach back and turn a knob and adjust the headrest to put it in position.