The first term in the Chinese sexagenary cycle is Jiazi, written 甲子 and romanized jiǎzǐ.
Jiazi combines the first of the ten Heavenly Stems, jia, with the first of the twelve Earthly Branches, zi. The two repeating sequences align to create 60 valid stem-branch combinations, so Jiazi is term 1 and the cycle eventually returns to it after Guihai, term 60.
The system was used to name and record days in China long before it became a way to label years. Its earliest surviving attestations appear on Shang-dynasty oracle bones, dating to around the late second millennium BC. The same structure later influenced calendrical traditions in Japan, Korea, and Vietnam.
Jiazi is sometimes translated as “Wood Rat,” because the stem and branch also carry element and animal associations. That label is useful shorthand, but Jiazi is fundamentally a calendrical stem-branch name, not simply an animal-year name. Another common error is to assume every stem-branch pairing is possible; only 60 of the 120 theoretical combinations occur.