 This means the simultaneous use of two or three different calligraphic styles on coins of the same period title which are otherwise identical in size of hole, width of rim, thickness, size and position of the characters and alloy.One can assume that these congruences arose from the workmanship of the different mints, but no attributions have yet been proposed.From the beginning of the dynasty, iron coins were extensively used in present-day Sichuan and Shaanxi where copper was not readily available.