to monitor these provinces. Each of the provinces was divided into sub-provinces, each having its own governor, who was chosen either by the royal court or by the satrap. To assess tributes, a commission evaluated the expenses and revenues of each satrap. To ensure that one person did not gain too much power, each satrap had a secretary, who observed the affairs of the state and communicated with Darius; a treasurer, who safeguarded provincial revenues; and a garrison commander, who was responsible for the troops. Additionally, royal inspectors, who were the "eyes and ears" of Darius, completed further checks on each satrap. The imperial administration was coordinated by the chancery with headquarters at Persepolis, Susa, and Babylon with Bactria, Ecbatana, Sardis, Dascylium and Memphis having branches. Darius kept Aramaic as the common language, which soon spread throughout the empire. However, Darius gathered a group of scholars to create a separate language system only used for Persis and the Persians, which was called Aryan script and was only used for official inscriptions. Before this, the accomplishments of the king were addressed in Persian solely through narration and hymns and through the "masters of memory". Indeed, oral history continued to play an important role throughout the history of Iran. Economy Darius introduced a new universal currency, the daric, sometime before 500 BCE. Darius used the coinage system as a transnational currency to regulate trade and commerce throughout his empire. The Daric was also recognized beyond the borders of the empire, in places such as Celtic Central Europe and Eastern Europe. There were two types of darics, a gold daric and a silver daric. Only the king could mint gold darics. Important generals and satraps minted silver darics, the latter usually to recruit Greek mercenaries in Anatolia. The daric was a major boost to international trade. Trade goods such as textiles, carpets, tools and metal objects began to travel throughout Asia, Europe and Africa. To further improve trade, Darius built the Royal Road, a postal system and Phoenician-based commercial shipping. The daric also improved government revenues as the introduction of the daric made it easier to collect new taxes on land, livestock and marketplaces. This led to the registration of land which was measured and then taxed. The increased government revenues helped maintain and improve existing infrastructure and helped fund irrigation projects in dry lands. This new tax system also led to the