The Rat (鼠) is the first of the repeating 12-year cycle of animals which appear in the Chinese zodiac, constituting part of the Chinese calendar system (with similar systems in use elsewhere). The Year of the Rat in standard Chinese is (Chinese: 鼠年; pinyin: shǔnián); the rat is associated with the first branch of the Earthly Branch symbol 子 (zǐ), which starts a repeating cycle of twelve years. The Chinese word shǔ (鼠) refers to various types of Muroidea, such as rats and mice. The term "zodiac" ultimately derives from an Ancient Greek term referring to a "circle of little animals". There are also a yearly month of the rat and a daily hour of the rat (Chinese double hour, midnight, 11:00 p.m. to 1:00 a.m.). Years of the rat are cyclically differentiated by correlation to the Heavenly Stems