It's the logical consequence of a 1-5 series with denominations varying by 10ˆx, where x is zero or a whole number. A 1-2-5 series would yield 888 for x = 0, 1, 2. Example: if there were a five euro coin, the denominations of the euro would add up to 888 cent (1-2-5-10-20-50-100-200-500). Include banknotes and you can take it to €888.88 without cheating. No such regularity for fractions, like a 1-2½-5 series, because the halves get in the way. The Soviets and Soviet Bulgarians couldn't always do it either with their 1-2-3-5 series because for x = 0, the total is more than 10, but they could make a symmetric number, e.g. for x = 0, 1, 2: 1221 kopeks.

Peter