The longer slot with five beads below the Ө position allowed for the counting of 1/12 of a whole unit called an "uncia" (from which the English words "inch" and "ounce" are derived), making the abacus useful for Roman measures and Roman currency. The first column was either a single slot with 4 beads or 3 slots with one, one and two beads respectively top to bottom. In either case, three symbols were included beside the single slot version or one symbol per slot for the three slot version. Many measures were aggregated by twelfths. Thus the Roman pound ('libra'), consisted of 12 ounces ("unciae") (1 uncia = 28 grams). A measure of volume, "congius", consisted of 12 heminae (1 hemina = 0.273 litres). The Roman foot ("pes"), was 12 inches ("unciae") (1 uncia = 2.43 cm). The "actus", the standard furrow length when plowing, was 120 "pedes". There were however other measures in common use - for example the "sextarius" was two "heminae".