Abacus lets the learner explore different representations of numbers using
different mechanical counting systems developed by the ancient Romans and
Chinese. There are several different variants available for exploration: a
suanpan, the traditional Chinese abacus with 2 beads on top and 5 beads below;
a soroban, the traditional Japanese abacus with 1 bead on top and 4 beads below;
the schety, the traditional Russian abacus, with 10 beads per column, with the
exception of one column with just 4 beads used for counting in fourths; and the
nepohualtzintzin, the traditional Mayan abacus, with 3 beads on top and 4 beads
below (it uses base 20).
