Abacus is the Latin word for a square board.

Here's a picture of the abacus on top of a column:

http://www.pitt.edu/~medart/image/glossary/abacus.jpg

The calculating abacus was a square board upon which stones (calculi) were used to perform arithmetic functions:

http://www.addiator.de/abacus-vom-abacus-zum-computer.jpg

Well, sort of square.