The Abacus and the Numeral Frame - The Japanese Abacus
Most recent Japanese abaci (soroban) have one counter above and four below. Some older Japanese instruments have five counters in the lower section. The right-most column represents units, the next tens, the next hundreds, etc. For numbers with digits to the right of the decimal point, the first column is for the smallest decimal term. In multiplication, some columns are used for the number being multiplied and some for the product. In part for this reason, there are more columns on most abaci than are usually used in addition. In any calculation, the counters that represent numbers are those moved against the crossbar.