Over time, the key holes of a violin widen, and the keys narrow.
Eventually there is no more room on the taper of the key to fill the
hole and hold the string in place through friction.  At this point, a
violin maker drills out the hole, plugs it, drills the plug, makes a
new key and installs it.  Good to go for another generation...or not,
depending on the frequency of the fiddling.

Is this not like a newel post?  Or a spindle?  How difficult is it to
take a stair rail apart and remake the worn joints?

-jc

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