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?