Cowrie shells, or Bia, were used for small denominations in Thailand for hundreds of years. They were brought into Sukhothai by oversea merchants from the Maldive Islands in the Indian Ocean. There were 8 different types of cowrie shells: Bia Plong, Bia Kaa , Bia Chan, Bia Nang, Bia Moo, Bia Pong Lom, Bia Bua and Bia Tum. Bia Chan was of regular size and was widely used in China and countries in Africa since the ancient time.
Pod Duang was made from a piece of a small silver ingot with weights varying considerably according to size. This type of money was unique, using silver manually formed into eliptical bars with both ends pressed inwards and hammered into shape resembling a Duang or a worm called "Ngoen Klom" or "Ngoen Pod Duang" or "Bullet Money".
Generally there were 2 marks stamping on Pod Duang, one of which was constant, representing the Dynasty's Mark or Kingdom's Mark. The other varied and represented the personal emblem of the reigning monarch.. During the Sukhothai period, most pieces bore more than two marks such as Ratchawat ( Pyramid of dots), an elephant, the Wheel of Law (Buddhist Symbol), etc.