The floor plan for the Japanese abacus (soroban) is one bead above, four beads below. The floor plan for the Chinese abacus (ch'uan pan) is two beads above, five below. The user's manual I had for my soroban said that the fifth bead was "of practical use but abstract convenience." I always suspected that line of having a bit of a typo in it or possibly just a bad translation (the opening line was "Soroban Japanese calculator are very much simply constructed"). The Chinese abacus is normally built on a larger scale than the Japanese.