In-Yo/Yin-Yang Disc

In Chinese philosophy, yin and yang (/jɪn/ and /jɑːŋ, jæŋ/; Chinese: 陰陽 yīnyáng, lit. "dark-bright", "negative-positive") is a concept of dualism in ancient Chinese philosophy, describing how seemingly opposite or contrary forces may actually be complementary, interconnected, and interdependent in the natural world, and how they may give rise to each other as they interrelate to one another.

The concept entered Japan in early times as in-yō. A government bureau existed in Japan as early as 675 ce to advise the government on divination and on control of the calendar according to in-yō principles, but it later fell into disuse. In-yō notions permeated every level of Japanese society and persist even into modern times, as evident in the widespread belief in lucky and unlucky days and directions and in consideration of the zodiac signs when arranging marriages.