type of tortoise used for divination
蔡 centers on a specific type of tortoise used in ancient divination rituals. The meaning is narrow and refers to this particular ritual object rather than a broad concept.
蔡 combines 艹 (grass) with 祭 (ritual), likely referring to a type of tortoise used in divination, perhaps named after a place or associated with ritual grass. The exact historical development is uncertain.
Picture a ritual 祭 where a special tortoise shell is placed on a bed of grass 艹 for divination. 蔡 is that ritual tortoise.
For サイ, imagine a sigh (サイ) of reverence as the diviner places the tortoise on the grass for the ritual.
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