The text of the
Book of Changes is a set of oracular statements represented by 64 sets of six lines each called
hexagrams (卦
guà). Each hexagram is a figure composed of six stacked horizontal lines (爻
yáo), each line is either
Yang (an
unbroken, or
solid line), or
Yin (
broken, an
open line with a gap in the center). With six such lines stacked from bottom to top there are 26 or 64 possible combinations, and thus 64 hexagrams represented.
The hexagram diagram is composed of two three-line arrangements called
trigrams (卦
guà). There are 23, hence 8, possible trigrams. The traditional view was that the hexagrams were a later development and resulted from combining the two trigrams. However, in the earliest relevant archaeological evidence, groups of numerical symbols on many Western Zhou bronzes and a very few Shang oracle bones, such groups already usually appear in sets of six. A few have been found in sets of three numbers, but these are somewhat later. Note also that these numerical sets greatly predate the groups of broken and unbroken lines, leading modern scholars to doubt the mythical early attributions of the hexagram system.
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Each hexagram represents a description of a state or process. When a hexagram is cast using one of the traditional processes of
divination with I Ching, each of the yin or yang lines will be indicated as either
moving (that is, changing), or
fixed (that is, unchanging). Moving (also sometimes called "old", or "unstable") lines will change to their opposites, that is "young" lines of the other type—old yang becoming young yin, and old yin becoming young yang.
The oldest method for casting the hexagrams, using
yarrow stalks, is a
biased random number generator, so the possible answers are not equiprobable. While the probability of getting either yin or yang is equal, the probability of getting old yang is three times greater than old yin.
