The most common theory on the arrangement of oarsmen in the new ship types is that of " double-banking ", i . e ., that the quadrireme was derived from a bireme ( warship with two rows of oars ) by placing two oarsmen on each oar, the quinquereme from a trireme by placing two oarsmen on the two uppermost levels ( the " thranitai " and " zygitai ", according to Greek terminology ), and the later hexareme by placing two rowers on every level.
