We happen to have a cute little baby at home, and some of her baby toys are cooler than others. Take for instance this great rattle of hers; I figure chances are pretty good that its designers didn't intend it to be a functioning, trinary, two-digit abacus, but that's exactly what it is. Check it out, I've taken a sequence of pictures showing how to use the thing to count from 0 to 8 in trinary.

Trinary is a number system like decimal or binary, but this one is base three. Like counting in decimal or binary, for each digit you start at zero and count up to N-1, where N is the number system base (three in this case). Think about it, that's what you do daily in our standard base-10 (decimal) system -- a given digit is between zero and nine, and once you count past nine you go back to zero and add one to the next digit. Same thing here in trinary. In any such number system you can count from zero up to N^m-1, where N is again the number system base and m is the number of digits. So just as in decimal where two digits will let you count from zero to 99 (=100-1, ie 10²-1), here on this trinary two-digit abacus we can count from zero to 8 (=9-1, ie 3²-1).

You can actually do arithmetic on an abacus too (as folks have done for centuries), but of course with base-3 and two-digits that's pretty limited. Unlike a base-10 abacus, however, this one makes for a nifty little car when set to the count of 4. Look again in the sequence!