Tax-exemption certificates for the clergy (dudie 度牒) were used during the Tang 唐 (618-907) and Song 宋 (960-1279) periods. Monks and nuns, but also any owner of such a type of certificate was spared payment of the land tax or labour for the community (yaoyi 徭役). The Buddhist clergy was registered in special household registers (senghu 僧戶) which were not used by the Ministry of Revenue (hubu 戶部). Tax exemption certificates were issued by the Ministry of Sacrifices (cibu 祠部), and were therefore also known by the name cibudie 祠部牒. State officials were allowed to purchase such certificates, thus profiting from the privilege of the clergy. The government deliberately allowed the trade with exemption certificates to profit from the revenue, which was particularly earmarked for military expenditure. In some cases, the government even allowed the extraordinary issuance of such certificates. During the Tianbao reign-period 天寶 (742-755), for instance, the Counsellor-in-chief Yang Guozhong 楊國忠 (700-756) dispatched Censor Cui Zhong 崔眾 to Taiyuan 太原, where he sold tax extemption certificates and yielded more than 10,000 cash. In 1067, the transport commission (zhuanyunsi 轉運司) of Shaanxi sold one thousand tax exemption certificates. The revenue was used for disaster relief. The Southern Song 南宋 (1127-1279), in dire need for higher revenue to finance their military campaigns against the Jurchens, largely expanded the business of dudie certificates. The Ming dynasty 明 (1368-1644) reorganized the system and demanded that purchasers travel to the capital and undergo a professional examination in religious matters. In this way, only educated monks and nuns would have the chance to obtain certificates. These were issued by two new institutions, the Central Buddhist Registry (senglusi 僧錄司) and the *Central Daoist Registry (daolusi 道錄司). The sales of clergy certificates was stopped in 1645, but was revived in 1736 on a small scale.