I wrote: "That's typically 21 columns (divided into registers) with 6 beads per column. The top bead is a binary 2, and the total is 5 * 2 = 10 per column, with 2 or 3 registers per abacus."
Whoops. I can't count. I mean there are 4 beads on the bottom and one binary on the top indicating a five, so the total in any column is 9. Count to 4 on the bottom, reset and set the top as 5, then 6 through 9 on the bottom again. At 10 you reset both and carry the 1. So, for example, to register a 7 you set the bead (5), and 2 bottom beads. Where "set" means "pushed up." I actually learned how to use the Japanese abacus with some proficiency many years ago. So I should have got that description right! There are some Chinese ones with different arrangements. - Jed
