According to the traditional "six books" style, what should the word "Yi" belong to?

It is also a single character and a single structure.

Yi (pinyin: y √) is a first-class Chinese character (commonly used word) of the common Chinese standard. This word first appeared in Shang Oracle Bone Inscriptions and Shang inscriptions. The ancient glyph indicates the position of two axils with left and right points, which should be the original word of "axil" and refer to axil. Later, "one" was used as an adverb.