The shi tian gan, ten heavenly stems, jia, yi, bing, ding, wu, ji, geng, xin, ren, gui, are the yin and yang poles of the five phases: yang wood, yin wood, yang fire, yin fire, yang earth, yin earth, yang metal, yin metal, yang water, yin water. Documented in oracle-bone usage for the Shang calendar, the stems originally named the ten-day week, xun, of the Shang ritual cycle, the days on which specific royal ancestors received offerings, the dead kings themselves taking stem-names, so that the calendar's alphabet began as a roster of the worshipped dead. Combined with the twelve branches they generate the sixty-jiazi cycle, the great clock of Chinese dating, and in BaZi each stem carries an element, a polarity, and a personality dossier the practitioners read like a type.
The folk note is the marketing: the twelve animals are memorable, the ten stems are homework, so the branches got the restaurants and the stems got the practitioners, yet every serious reading runs on stem interactions, combinations, clashes, transformations, the animals being the face and the stems being the engine. The tradition's own elegance: ten stems and twelve branches meshing at offset produce sixty unique pairs, the least common multiple doing what no designer could, a cycle long enough that no two years in a working lifetime need repeat, the calendar as a documented anti-repetition machine.
The claim: the stems' Shang origin is the archive's favorite calendrical secret, the time-system beginning as a death schedule, which ancestor eats on which day, so that every BaZi chart cast today is built from a language invented to feed kings, the personal reading riding on a liturgical substrate, your Day Master being, etymologically, a seat at the ancestors' table. Verdict: the archive files the documented origin and the pattern it fits, the archive's whole calendar wing running on the same discovery: the week, the year, the zodiac all begin as ritual rosters before they become personality quizzes. The ten stems fed the dead for a dynasty. Now they feed the readings. The cycle, per the tradition, does not care what it measures, and the dead, per the Shang, were the first to be measured by it. Check your stem. It was somebody's dinner day.
If the ten stems are the calendar's true alphabet, why did the twelve animals get the branding?