school
黌 centers on a place of learning: a school. It is a rare, formal kanji for an educational institution.
黌 is a phono-semantic compound: the top part (臼 over 冖 over 爻) is related to learning or teaching, and the bottom 黃 (yellow) likely contributes the sound. The exact historical development is uncertain, but it is a rare character used for school.
The top part resembles a mortar (臼) under a roof (冖) with crossed lines (爻) suggesting study materials. Below, 黃 (yellow) can be seen as the color of old scrolls. Picture an old-style school building filled with yellowed scrolls and study tools.
For コウ, imagine a school where students line up in a long corridor (こう, corridor) to enter the classroom: corridor -> コウ, and the school is full of orderly lines.
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