square character style, correctness
楷 centers on a formal, upright standard: the square, regular style of writing Chinese characters, and by extension correctness or model behavior.
楷 combines 木 (tree) with 皆, which likely contributes the sound. The character originally referred to a type of tree (Chinese pistache) whose wood was used for making model calligraphy tablets, leading to the extended meaning of a standard or model style.
A wooden 木 model 皆 used as a standard for correct writing: 楷 is the square, regular style that sets the standard for correctness.
For カイ, imagine a calligrapher named Kai who always writes in perfect square style: Kai -> カイ, and his work is the model of correctness.
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printed style (of writing Chinese characters); square style; block style; standard style
regular script (of Chinese characters); square style; block style; standard style
Chinese pistache (Pistacia chinensis)
square (block) style of printed typeface
printed, semi-cursive, and cursive scripts