Abacus (ἄβαξ, ἀβάκιον), a counting-board, the usual aid to reckoning in antiquity. The Egyptians, Greeks, and Romans alike used a board with vertical columns, on which (working from right to left) units, tens, hundreds, or (where money was in question) e.g. ⅛ obols, ¼ obols, ½ obols, obols, drachmae, sums of 10, 100, 1,000 drachmae, and talents were inscribed. When an addition sum was done, the totals of the columns were carried to the left, as in our ordinary addition. The numbers might be marked in writing or by pebbles, counters, or pegs.