～形
～けい · ～kei · JLPT N2 · noun
shape of ~
🔺 三角形 = three + corner + shape → triangle; ～形 tags the shape
Added to a noun, ～形 (kei) names the shape or form: 円形 is circle-shaped and 長方形 is a rectangle. The kanji's 彡 part is brush strokes, the lines that trace out a form. Also used in grammar: 過去形 is the past form.
形 — open frame 开 + hair/pattern 彡
形 combines 开 (originally a phonetic element suggesting the sound) with 彡, a radical depicting fine lines, hair, or decorative strokes. The 彡 part conveys the idea of visible markings or surface appearance, giving 形 its core meaning of 'shape' or 'form.' Over time it broadened to cover physical figure, appearance, and abstract form.