八はっ角かく柱ちゅう

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Meanings

1

noun

octagonal prism

A prism with an octagonal cross-section. Used in geometry and technical contexts.

角angle, corner, square

柱pillar, post, cylinder

六ろっ角かく柱ちゅう
六角柱 is a hexagonal prism, while 八角柱 is an octagonal prism.
四し角かく柱ちゅう
四角柱 is a quadrangular prism (often a rectangular prism), while 八角柱 has eight sides.

Compound of 八角 (はっかく, 'octagon') and 柱 (ちゅう, 'pillar, prism').