During the Northern and Southern Song dynasties,coins graded as "Xiaoping", "Zhe-er", and "Dangshi" were all cast with reign marks of emperors. Large numbers of "Duiqian" coins were also produced. Iron coins together with bronze ones were first circulated in the Southern Song dynasty and tiron coin casting reached its peak during Jia Ding Reign of the Southern Song dynasty.In the Southern Song dynasty,excessive release of paper notes led to their devaluation, so the imperial government in Lin'an (today's Hangzhou) later used bronze and lead plate coins to replace paper notes.Meanwhile, the Liao, Jin and Xixia dynasties had their own currency systems and cast round-with-a-square-hole coins imitating the shape of the Song coins.