The Chinese zodiac combines twelve earthly branches and animals with a ten-stem cycle of Yin and Yang forms of Wood, Fire, Earth, Metal and Water. Together they create a repeating sixty-year cycle: a full year name such as Yang Fire Horse returns once every sixty years.
The twelve animals are really the twelve earthly branches(地支) — fixed positions on a repeating cycle, each with an animal attached. Their fixed positions make the traditional groupings geometric: animals four apart form a trine, while animals six apart oppose each other.
The branches pair with ten heavenly stems (天干), which carry the five phases in Yin and Yang form. Ten and twelve do not divide evenly, so the same stem meets the same branch only once every sixty years. That is the sexagenary cycle, and it is why a sixtieth birthday is the traditional milestone: it is the first time a person's complete year name returns.
Because twelve is even, each animal always takes the same polarity — a Rat year is Yang without exception, an Ox year Yin. What varies across a lifetime is the element, which advances one phase every twelve years: a Rat born in 1984 belongs to a Wood year, the next Rat year is Fire, then Earth, Metal and Water before Wood returns.
A zodiac year begins at Chinese New Year rather than 1 January. January and early-February birth dates therefore need to be checked against that year's exact boundary.