 ", half-space " ", space " ", underbar "_" (as in maritime "21_450") or apostrophe «'».In some countries, these "digit group separators" are only employed to the left of the decimal separator; in others, they are also used to separate numbers with a long fractional part.An important reason for grouping is that it allows rapid judgement of the number of digits, via subitizing (telling at a glance) rather than counting – contrast with 100000000 for one hundred million.