Darius used a system of satrapies to unify his large empire under one government. A satrapy was a province ruled by a governor called a satrap. The satraps were appointed by the king and were responsible for collecting taxes, administering justice, and maintaining order in their provinces. They were also responsible for raising troops for the king's army. The satrapies were divided into smaller districts called hyparchies, which were ruled by hyparchs. The hyparchs were appointed by the satraps and were responsible for the day-to-day administration of their districts.