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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?