This article is from the Woodworking FAQ Collection 1, by multiple authors.
One of the most frequently asked of all frequently asked
questions. It rivals the everpresent question concerning
aftermarket TS fences, it lacks, however, the longevity of
threads engendered by that question.
Many answers have been suggested: it is used to test the temper
of the steel, to start the cut, to remove splinters from the
kerf, to hold a string for level, to hold string to tie on a
sawblade protector, repressive desublimation, you name it.
The most agreed upon answer is that it is a vestige of earlier
ornate decoration on European saw backs. According to a 1915
publication of Henry Disston & Sons (who should know something)
the nib is purely decorative.
 
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