This refers to the fundamental oracular calculation. The term “fundamental calculation,” which is from The Book of Changes, refers to the procedure for creating hexagrams by using fifty yarrow stalks. The diviner would repeatedly combine and separate groups of yarrow stalks according to a set of rules. The changes in the numbers of the stalks were used to ascertain the yin and yang, namely, the negative and positive, characteristics of each trigram, and the trigrams were in turn used to form a hexagram. This method of calculation was believed to represent the process through which everything was created and evolved, and it was used to explore and understand the rules which govern changes and their underlying trends.

The number (of yarrow stalks) in the fundamental calculation is fifty, of which forty-nine are used. The Book of Changes