As Karl Menninger says on page 315 of his book, "For more extensive and complicated calculations, such as those involved in Roman land surveys, there was, in addition to the hand abacus, a true reckoning board with unattached counters or pebbles. The Etruscan cameo and the Greek predecessors, such as the Salamis Tablet and the Darius Vase, give us a good idea of what it must have been like, although no actual specimens of the true Roman counting board are known to be extant. But language, the most reliable and conservative guardian of a past culture, has come to our rescue once more. Above all, it has preserved the fact of the "unattached" counters so faithfully that we can discern this more clearly than if we possessed an actual counting board. What the Greeks called "psephoi", the Romans called "calculi". The Latin word "calx" means 'pebble' or 'gravel stone'; "calculi" are thus little stones (used as counters)."