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 Ø is a letter, curse this arrangement).
(Interestingly, the slashed zero long predates computers; Florian Cajori's
monumental A History of Mathematical Notations notes
that it was used in the twelfth and thirteenth centuries.) 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. Old
CDC computers rendered letter O as an unbroken oval and 0 as an oval broken
at upper right and lower left. 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
(this was endorsed by a draft ANSI standard for how to draw ASCII
characters, but the final standard changed the distinguisher to a tick-mark
in the upper-left corner). Are we sufficiently confused yet?