Ten-year cycles

Picture life as a long walk through changing country. Every ten years or so the scenery shifts: a stretch of forest, then open fields, then the coast. You're the same walker, but the weather around you changes.

In BaZi, each stretch is a 10-year cycle or decade (大運, dà yùn), and each one has its own pillar.

Where they come from

The decades grow out of your Month pillar. They step through the sixty pillars one at a time, one per decade, either forward or backward. If your Month pillar is 丙寅, going forward gives 丁卯, 戊辰, 己巳 and so on. Going backward gives 乙丑, 甲子, 癸亥.

Forward or backward

The direction comes from an old formula. It combines the Yang or Yin of your Year stem with the Traditional formula in your birth details, Male or Female.

Male with a Yang year, or Female with a Yin year: forward.
Male with a Yin year, or Female with a Yang year: backward.

That's the only reason the app asks. It's used for the decades and the Kua number, and it says nothing about who you are.

When the first one starts

The app counts the time from your birth to the next solar term that opens a month (forward), or back to the last one (backward). Every three days count as one year. Fifteen days, for example, means the first decade starts at about five.

So the first decade starts at a different age for everyone, anywhere from infancy to about ten. Before it, the full reading says your Month pillar sets the tone; it calls that stretch "Early childhood". The dates are approximate, which is why the app writes "≈" and "about".

What you see in the app

Timeline, Decades: one arch per decade, with its pillar, the month and year it starts, "at age" and two themes. The themes are the roles its stem and branch play for your Day Master, like "Opportunity + Support". The one you're in says "Now".
Tap a decade: its sheet shows its two halves, how it meets your chart, your Day Master's stage, the stars it switches on and the years in it worth watching.
Life curve: below the arches, the app model's curve across your decades. It starts on; tap "Model layer: on" to hide it. It's the app's own weighting, not a verdict.
Today: "What surrounds you now" has a Decade card. When a new decade is a few months away, Today tells you.
Timeline, Years: the year a decade changes, a short note says so.

What a practitioner looks at in a decade

The two halves. In the Zi Ping school, the decade's stem weighs most in its first five years and its branch in the last five. Some schools give the branch more weight across the whole decade.
How it meets your chart. When the decade's branch combines or clashes with one of your branches, that pillar of yours stirs for those years.
Years that add up. When a year's pillar is the decade's own (歲運並臨), its theme feels doubled. When a year clashes the decade in both stem and branch (天剋地沖), it tends to shake things loose. The tradition says to take those years calmly, not fearfully.

Reading a change of decade

It's a change of scenery, not a cliff. The years around the change often feel like a transition, and the new tone settles in little by little.
Compare the themes. What's arriving with the new decade? What's leaving with the old one?
One decade isn't a grade. A higher stretch on the curve doesn't mean an easier life. Lower stretches are often when people build, learn or rest.

A good question for the change: what do I want to carry into the next stretch, and what can I leave behind?