the oval-shaped zero and circular letter o came into use together on modern character displays. the zero with a dot in the centre seems to have begun as a choice on ibm 3270 controllers (this has the problem that it looks like the greek letter theta). the slashed zero, looking like the letter o with a diagonal line drawn inside it, is used in old-style ascii graphic sets that came from the default typewheel on the well-known asr-33 teletype. this format causes problems because it looks like the symbol formula_1, representing the empty set, as well as for certain scandinavian languages which use ø as a letter.