Ancient Greek units of measurement varied according to location and epoch. Systems of ancient weights and measures evolved as needs changed; Solon and other lawgivers also reformed them en bloc. Some units of measurement were found to be convenient for trade within the Mediterranean region and these units became increasingly common to different city states. The calibration and use of measuring devices became more sophisticated. By about 500 BC, Athens had a central depository of official weights and measures, the Tholos, where merchants were required to test their measuring devices against official standards.