How it works
Take three matching coins and toss them together. Heads count 3 and tails count 2 (some schools swap them; what matters is sticking to one). Add up the three and you get one of the line values:
- 6: three tails. Old yin, a moving line.
- 7: two tails and a head. Young yang, a stable line.
- 8: two heads and a tail. Young yin, stable.
- 9: three heads. Old yang, moving.
Toss six times and write each line from the bottom up: the first toss is the bottom line. That gives you the primary hexagram and, if any lines move, the relating hexagram.
The odds
Each coin is fifty-fifty, so 6 and 9 each come up one time in eight, and 7 and 8 three times in eight. With the yarrow-stalk method the numbers shift: old yang turns up more often than old yin. Neither is more correct; they're different traditions.
Where it comes from
It's been in use at least since the Song dynasty (10th to 13th centuries), and some trace it further back. It won out because it's quick: in a couple of minutes you have your hexagram.
An example
If you get 7, 8, 9, 7, 8, 8, the third line is a moving yang. You read the hexagram, that line, and the hexagram it changes into. Step by step in How to Toss the Coins.