The oldest depictions of planes I know of are found on silver Roman coins dating from the first century BC, right as Rome was developing from a republic to an empire. The planes appear as symbols known as control marks. Control marks were likely used to identify the individual dies used to strike the coins; animals, tools, musical instruments and other day-to-day items were commonly depicted. The planes are simplified renditions with few details, but each clearly show the overall shape of the plane and raked irons. As the historian Roger B. Ulrich notes, "The fact that variants exist in these depictions is an indication that the tool already existed in diverse shapes and sizes and that it was a common enough tool to be recognizable for what it was even when rendered schematically."