Zǐ (子)

Zi (子) is the first of the Twelve Earthly Branches. Before anything else, the distinction that matters most: Zi is the branch — a calendrical unit — and the Rat is the popular Chinese-zodiac (生肖) animal that corresponds to it. Zi is not 'the Rat'; it is Zi, the first Earthly Branch, and the Rat is its familiar label. The branch is the technical thing a chart actually uses; the animal is the popular shorthand.

As a branch, Zi marks the dead of night and the deep of winter — midnight, the 11th lunar month, the still point where the old year's energy is exhausted and the new is not yet visible. Its element is Water at its coldest and most concentrated, everything held in potential. It is the seed in frozen ground: nothing moving on the surface, everything gathered underneath.

Every Earthly Branch is first a unit of time. Zi rules the hours 23:00–01:00 (the 子時 double-hour), the 11th lunar month around the winter solstice, and the north of the compass. A chart reads a branch by where it sits — year, month, day, or hour pillar — because that placement is literally a coordinate in time.

An Earthly Branch carries its elements through hidden stems (藏干) — Heavenly Stems concealed inside it. Zi is the purest of the branches: it hides only Yin Water (癸). That single, unmixed hidden stem is why Zi reads as concentrated, undiluted Water — there is no other element buried in it to complicate the signal.

Zi's clash (六沖) is with Wu (午, the Horse) — the deep-winter / high-summer, Water-against-Fire axis. It joins the Water trinity (三合) 申子辰 (Shen–Zi–Chen), where it is the centre and strongest member. Its six-combination (六合) is with Chou (丑, the Ox), the pair said to combine toward Earth. These are structural relationships between positions in the chart, not predictions of events.