0: Numeric Zero, As Opposed To The Letter `O' (the 15th Letter Of The English Alphabet).

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:0: Numeric zero, as opposed to the letter `O' (the 15th letter of
the English alphabet). In their unmodified forms they look a lot
alike, and various kluges invented to make them visually distinct
have compounded the confusion. If your zero is center-dotted and
letter-O is not, or if letter-O looks almost rectangular but zero
looks more like an American football stood on end (or the reverse),
you're probably looking at a modern character display (though the
dotted zero seems to have originated as an option on IBM 3270
controllers). If your zero is slashed but letter-O is not, you're
probably looking at an old-style ASCII graphic set descended from
the default typewheel on the venerable ASR-33 Teletype
(Scandinavians, for whom slashed-O is a letter, curse this
arrangement). If letter-O has a slash across it and the zero does
not, your display is tuned for a very old convention used at IBM
and a few other early mainframe makers (Scandinavians curse
*this* arrangement even more, because it means two of their
letters collide). Some Burroughs/Unisys equipment displays a zero
with a *reversed* slash. And yet another convention common on
early line printers left zero unornamented but added a tail or hook
to the letter-O so that it resembled an inverted Q or cursive
capital letter-O. Are we sufficiently confused yet?
-- The AI Hackers Dictionary