The decree called "unifying the writing" was promulgated during the Qin dynasty, which demanded that all seal inscriptions and knob styles be unified. Rigorous rules and regulations of using raw materials and styles were formulated according to various official ranks. "Mou Zhuan" became the exclusive style for seals. The official seal system was matured during the Qin and Han dynasties and lasted for eight hundred years. Seal production reached its climax during this period. Rigid formats were set up for official and private seals in order to reveal characteristics of steadiness and harmony.