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?