- the japanese abacus has 4 beads, 1 to 4 and then another bead seperated by a wire. This seperate bead equals 5 units. So total is 9. Rhere are many rows of these, so you can calculte huge numbers with many decimals on a relatively small abacus.

So, this system (Soroban, several good applets on the internet)is also used on the hands, where the fingers on one hand are 1 to 4 and the thumb is 5 units, total 9. The other hands symbolises 10, 20, to 90.

Total is 99.

Works very well, kids in China learn it, and is a lot better than using your fingers to count to 10:

Hold out all 10 of your fingers (let thumbs be fingers here)
if you are multiplying 9 by (example) 6, hide your 6th finger.
Then, you will be left with 5 on the left side, and 4 on the right side. Yealding the answer: 54.

Another example, 9x8, hide the 8th finger, you're left with: 7 on the left, and 2 on the right. Answer: 72.

etc...

Unfortunately, this only works on 9 x (number) and the (number) is between 1-10 inclusive)