Ten years, one pillar
Da Yun cuts a life into ten year slices, and each slice carries a pillar. A pillar, here, is what it is everywhere else in the system: a heavenly stem set on an earthly branch. Those pillars are not chosen case by case, they are taken from the sexagenary cycle, the closed sequence of the sixty valid combinations.
The distinction to hold on to is simple, and it is what prevents misreadings. The birth chart never moves. The eight characters set at the instant of birth stay the same from one end of a life to the other. Da Yun does not correct them, does not replace them and does not cancel them. The decade lights up a period, it does not rewrite the chart.
A reading posture follows from that. A Da Yun decade announces nothing and dates nothing. It shows in which tone a period presents itself to a given chart, and that tone only means something when brought back to the eight starting characters. Two people going through the same decade pillar do not go through it in the same way, since the chart it lands on differs. That is what separates Da Yun from anything resembling a universal calendar, where the same period would hold good for everyone.
The sequence starts from the month pillar
The starting point is neither the year pillar nor the day pillar: it is the month pillar. That detail is nothing incidental, it commands everything that follows. Two charts sharing the same birth year but not the same month do not have the same series of decades.
That dependence on the month pillar has a practical consequence. The month pillar is the one that switches along with the solar seasons, and not on the first day of the civil month: it is determined through true solar time and the calendar switches. A birth falling very close to one of those switches does not only change one character of the chart, it shifts the entire decade series. This is the point where an approximate date costs the most.
The direction of travel, forward or backward
Once the starting point is fixed, it remains to be known in which direction the cycle is travelled. Two crossed pieces of data decide it: the polarity of the stem of the birth year and the sex of the person.
The rule holds in one sentence. The sequence runs forward if the stem of the birth year is Yang for a man, or Yin for a woman. In the two other cases it runs backward.
An example makes the rule readable. A year carrying the stem Jia, which is a Yang stem, will give an ascending sequence for a man and a descending one for a woman, all else being equal. The birth chart itself is rigorously the same in both cases: only the order of travel changes.
One clarification is needed, because this rule is often misreported. Yang and Yin are not genders and do not become genders here. They are two phases, and the calculation convention crosses them with sex to determine a direction, without either polarity being superior to the other, or assigned to anyone.
One notch per decade
The movement itself is the simplest part of the whole procedure: each decade moves one notch forward or backward in the cycle of sixty. There is no jump, no shortcut, no exception. The sequence moves step by step, stem after stem and branch after branch, in the direction fixed above.
Over a lifetime, that amounts to a handful of consecutive pillars, that is to say a small fraction of the complete cycle. The sixty pillars are never travelled in full within one existence: Da Yun is only a segment of the great wheel, travelled one way or the other from a point that depends on the month of birth.
The starting age, an acknowledged simplification
There remains the question of entry into the first decade. In the classical method, that age depends on the distance between the moment of birth and a solar term: the closer or further the birth is from that marker, the earlier or later the entry.
The current calculation does not make that distinction. It fixes the starting age at 8 years for every chart. This is an approximation, and it is acknowledged as such: the value used is close to the average, it is not the result of the full calculation.
The boundaries of the decades therefore have to be read for what they are, markers and not dates. A decade boundary shown on screen is not a threshold accurate to the day, and it would be dishonest to present it that way. Anyone wanting to work on their decade transitions as precisely as possible should know that this step of the calculation is simplified, rather than believe in a precision that is not there.
What Da Yun is not
Da Yun is not to be confused with the year animal, which uses only one character out of eight and says nothing about the decades. Nor does it produce dated prediction: the procedure describes a succession of tones, not a series of events. Its value is to make plain that a chart is not read as one block, but by periods, each of them set on the same motionless base of eight characters.
