The 10 Heavenly or Celestial Stems (天干, Mandarin tian1gan1, Japanese tenkan) combine with the 12 Earthly Branches to form the traditional 60-year Chinese calendar. While this calendar is obsolete for all official use, it lives on in Chinese astrology, and the 12-year cycle of the Chinese zodiac ("Year of the Dragon" etc) is in fact determined by mapping the horoscope signs on top of the 60-year cycle.

The more interesting and current use of the Heavenly Stems is that they form a rudimentary Chinese alphabet in the very limited sense that they can be used to rank or order items, as in English terms like "Plan A", "Site B" and "Group C". The system remains in heavy use in China but is rarely seen in Japan, where the homegrown iroha ordering of kana is usually preferred, although this ordering -- called kouotsuhei after the readings of A, B and C -- is still occasionally used for formal titles, placenames, and the like. The full sequence goes like this:

甲  1  A  jia3   kou
乙  2  B  yi3    otsu
丙  3  C  bing3  hei
丁  4  D  ding1  chou
戊  5  E  wu4    bo
己  6  F  ji3    ko
庚  7  G  geng1  kou
辛  8  H  xin1   shin
壬  9  I  ren2   nin
癸 10  J  gui3   ki
Order: Unicode, numeral, letter, Mandarin reading, Japanese reading

What is odd about this sequence of characters is that they are essentially meaningless. While they of course all do have historical meanings, there is no rhyme or reason to the order -- the first four are "armor", "strange", "fire" and "street" -- and in modern times they are rarely used for purposes other than ranking, the unlucky number 4 (丁, a counter for blocks) and the lucky 8 (辛, spicy/bitter) being the exceptions. For precisely this reason these characters are also a part of the set used for phonetic spelling in both China and Japan.

I wonder how many people spotting the title thought this had something to do with marijuana?

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