Why Are Qianlong Tongbao Coins Used for I Ching Divination?
The short answer first
Casting a hexagram makes no demands on the coins. Three coins of the same kind, with sides you can tell apart, are enough. A hexagram cast with modern coins is no different from one cast with old coins. What follows is about custom, not about rules.
Why people like to use them
- They are plentiful: the Qianlong emperor reigned for sixty years and coins were minted in enormous quantities. Many survive, so it is easy to put together three in similar condition.
- They are easy to recognize: the front carries the four Chinese characters "Qianlong Tongbao", read top to bottom and then right to left. The back carries Manchu script: on the left the word bao ("treasure"), and on the right the name of the mint (for example Baoquan or Baoyuan). The two sides can be told apart at a glance.
- The copper is good, and the size and thickness sit well in the hand. They ring clearly when thrown.
- A lucky association: Qian is the first of the sixty-four hexagrams, and "Qianlong" also suggests flourishing and prosperity. This is a folk notion and has nothing to do with the reasoning behind divination.
As for "Five Emperors coins" and similar practices for protecting a house, they belong to a different strand of folk custom and have nothing to do with divination by the Zhouyi.
Which side is yang
This site counts the back (the side with Manchu script) as 3, which is yang, and the inscribed side (the side with "Qianlong Tongbao") as 2, which is yin. Some people set it the other way around. Either works, as long as you decide before you throw and do not change it during the six throws. For details, see How to cast with three coins.
The three coins thrown on this site have "Qianlong Tongbao" (乾隆通寶) in regular script on the front, and the Manchu script on the back is taken from a rubbing of a Qianlong Tongbao coin.