How can an abacus add numbers with only a few beads?

A Japanese soroban represents digits by moving beads toward a central beam. On each rod, four lower beads each stand for one, and one upper bead stands for five. Beads moved away from the beam do not count.

The rods give these bead patterns place value. On a chosen ones rod, the upper bead and two lower beads show seven. On the tens rod, the same pattern would show seventy. Position changes value even though the beads look alike.

Consider eight plus five. Eight is five and three on the ones rod. The result, thirteen, is one ten and three ones. Clearing the upper five-bead and adding one bead on the tens rod changes eight into thirteen: ten was added and five removed, a net increase of five.

The answer stays visible in the bead positions without batteries or written digits. The soroban works because its movements preserve numerical value. It makes the exchanges hidden inside written arithmetic into changes you can see.