All merchants use private cipher marks to note cost or selling price of goods. The cipher is usually made up from some short word or sentence of nine or ten letters, as:

Cornelius, A. 123456789 0. Five dollars, according to this key, would be eaa. But generally an extra letter is used to prevent repeating the mark for 0. If the sign lor a second 0 in this case were y, we would have eay instead of eaa.