Heavenly Stems and Earthly Branches: China's 60-Cycle System

Chinese timekeeping runs on two interlocking sets of signs: ten heavenly stems (天干, tiangan) and twelve earthly branches (地支, dizhi) — together covered by the umbrella term ganzhi (干支), and in older romanizations tien gan dizhi. Neither set is used alone. One stem and one branch, taken in step, form a two-character label — 甲子 jiazi, 乙丑 yichou, 丙寅 bingyin… — and because the two sets have different lengths, the labels run through sixty distinct combinations before returning to the start. That is the Chinese sexagenary cycle: the sixty-term counter that dates years, months, days, and hours on a Chinese calendar, and that supplies all eight characters of a Bazi chart. This page explains the two sets of signs, how the pairing produces sixty terms rather than a hundred and twenty, and where the system actually came from — a story that begins with carved oracle bones, not with books.

The ten heavenly stems: element and polarity

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The stems are the simpler half of the system. They are not ten things but ten labels — the five elements (五行, wuxing) each split into a yang and a yin representative, five times two making ten:

The images in the last column are the ones Bazi primers traditionally assign; the structural facts are the first four columns. Polarity alternates by position — odd positions yang, even positions yin — so 甲, 丙, 戊, 庚, 壬 are the five yang stems and 乙, 丁, 己, 辛, 癸 the five yin stems. When a Bazi reader speaks of “jiǎ wood” (甲木), the polarity is already inside the name: 甲 is yang wood, and 乙 is its yin counterpart. Within a chart, stems also enter defined relations with one another (for example the five stem combinations, such as 甲 with 己 tending toward earth) — territory that matters once you read charts, and the ten gods are built from exactly this kind of stem-to-stem comparison.

The twelve earthly branches: animal, hours, and solar months

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The branches carry more freight than the stems. Each of the twelve names an animal, a two-hour span of the day, an element, and — less known outside China — one of the twelve solar months of the year:

The animal column is the popular face of the branch — the zodiac, with a full year-by-year table in Chinese Zodiac Signs by Year. The month column deserves a pause, because it hides the system’s chief beginner trap: branch-months are cut by the Sun, not the Moon. Each month branch begins at a jie (节) solar term, and the year was made to start at Lichun (立春, Start of Spring, February 3–5) — so the sequence of month branches begins with 寅, the third branch, and runs 寅, 卯, 辰 … on to 丑. 子 month is not a beginning at all: it is the block containing the winter solstice, roughly December 7 to January 5. The full mechanics of the term boundaries are laid out in The 24 Solar Terms.

One further property, kept brief because it only matters inside chart reading: each branch is said to hide one to three stems within it (地支藏干), so a branch is less a pure element than a bundle of them. Reading those hidden stems is Bazi work — the entry point is What Is Bazi.

Why the cycle is 60 and not 120: the pairing arithmetic

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Here is the question this page exists to answer. Ten stems and twelve branches suggest 10 × 12 = 120 possible pairs, yet the famous cycle has only sixty terms. The other half never occurs, and the reason is a single rule:

A stem only ever pairs with a branch of the same polarity. The five yang stems (甲丙戊庚壬) pair with the six yang branches (子寅辰午申戌); the five yin stems (乙丁己辛癸) with the six yin branches (丑卯巳未酉亥). That is 5 × 6 = 30 yang pairs plus 5 × 6 = 30 yin pairs — exactly sixty.

The same fact seen from the mechanics: picture two gear wheels fixed to the same shaft, one with ten teeth (the stems), one with twelve (the branches), advancing one notch apiece per unit of time. The wheels realign at the least common multiple of 10 and 12, which is 60 — and because parity is preserved at every step (an odd-position stem always faces an odd-position branch), each of the sixty admissible pairs appears exactly once per revolution. The sixty inadmissible pairs — 甲 with 丑, 甲 with 卯, 乙 with 子 … every mixed-polarity combination — are mechanically impossible.

The first twelve terms of the cycle show the wheels at work:

Term 11 is the tell: the stems ran out at 癸 (10), so that wheel wraps to 甲 while the branches continue on to 戌. Two terms later the branch wheel comes round too, and 丙子 (13) opens the next pass. Neither wheel waits for the other — they simply keep stepping, and term 60 is 癸亥, after which term 61 is 甲子 again: one full liushi jiazi (六十甲子), the “sixty jiazi”, the cycle’s proper name taken from its first term. In year terms, 1924, 1984, and 2044 are all 甲子 years; the century-long year-by-year table lives in Chinese Zodiac Signs by Year.

A corollary worth keeping: because of the parity lock, any given stem only ever meets six of the twelve branches. 甲 teams with 子, 寅, 辰, 午, 申, 戌 — and never once with 丑, 卯, 巳, 未, 酉, or 亥, in any of the sixty terms.

Years, months, days, hours: what the cycle labels

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The cycle’s whole job is to stamp time. Each of the four units of a moment gets one stem-branch pair, called a pillar (柱):

Four pairs make eight characters — Chinese 八字, “eight characters”, the source of the English name Four Pillars of Destiny. What those eight characters then mean is the whole art of Bazi, laid out from zero in What Is Bazi.

Two boundary facts do most of the practical damage. First, the year does not change over at midnight on December 31st, nor at Chinese New Year, but at Lichun — so a January or early-February birth usually belongs to the previous year’s sign (worked through with examples in Chinese Zodiac Signs by Year). Second, the zi hour runs 23:00–01:00 and straddles midnight, so a birth near 23:00 or 00:00 can turn both the day and the hour pillar. Schools differ on whether the day pillar changes at 23:00 or at midnight, which is precisely why near-midnight births need the exact clock time — and a stated convention.

For a specific date, the Chinese Almanac Calendar shows the year, month, and day pillars in force on that day (a calendar date carries three pillars; the hour pillar requires a birth time). For a full chart from an exact birth moment, the Bazi Calculator lays out all four pillars at once.

Where you meet stems and branches in practice

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You have almost certainly met this system already, under one of these guises:

• Zodiac years. “Year of the Dragon” is the animal face of a branch. The full sign is the stem-branch pair, and the element in phrases like “Wood Dragon” (2024) comes from that year’s stem — the layering is explained in Chinese Zodiac Signs by Year.
• History-book event names. Chinese historiography dates events by their stem-branch year: the First Sino-Japanese War of 1894 is the Jiawu War (甲午), the 1898 reform the Wuxu Reform (戊戌), the 1900 Boxer indemnity the Gengzi Indemnity (庚子), the 1911 revolution the Xinhai Revolution (辛亥). To a reader fluent in the cycle, the names are self-dating — 辛亥 is 1911.
• The almanac. Auspicious-date selection (择日) reads the stem-branch pillars of candidate days against the event and the person; every page of a traditional almanac is dense with ganzhi labels. The Chinese Almanac Calendar shows any day’s pillars directly.
• Acupuncture hours. Traditional Chinese medicine maps the twelve meridians onto the twelve shichen (the practice of ziwu liuzhu, 子午流注), which is why some acupuncture schedules still refer to two-hour periods rather than clock hours.

The calendar underneath all of this — how the 60-cycle stays in step with lunar months and leap years — is a separate machine, covered in How the Chinese Lunar Calendar Works.

Where the system came from

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The earliest evidence for the stems and branches is not a book — it is bone and shell. In the oracle-bone corpus of the late Shang capital at Yinxu (modern Anyang), inscribed roughly 1250–1046 BCE, every divination record is dated by a ganzhi day, and among the surviving inscriptions are complete sixty-term tables carved as reference calendars: the full cycle, in its fixed order, on turtle plastrons and ox shoulder-blades more than three thousand years old. The sequence on those bones is the same sixty terms, in the same order, in use today. Some scholars argue the ten stem characters began as the names of Shang royal ancestors; the argument is built on those same inscriptions and remains debated.

The cycle was a day counter first. Its oldest independent check is astronomical: the Spring and Autumn Annals (《春秋》) records a solar eclipse on a jisi (己巳) day in 720 BCE — the third year of Duke Yin of Lu — and modern computation confirms the eclipse; the day label matches the continuous count, evidence that the sixty-day cycle has run without interruption for the twenty-seven centuries since (Western Zhou bronze inscriptions keep it attested in between). Using the cycle to label years settled into standard practice later, by Han times; the Hanshu’s Treatise on the Pitch-pipes and Calendar (《汉书·律历志》, first century CE) is the classical record of that machinery, with the sexagenary cycle embedded in the imperial calendar computations.

A short pinyin glossary

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English texts on this subject mix pinyin, Wade-Giles-style spellings, and bare romanization. The vocabulary is small:

Further reading: How the Chinese Lunar Calendar Works · The 24 Solar Terms · Chinese Zodiac Signs by Year · What Is Bazi · Yin and Yang · Back to the Foundations section.