ellipse
橢 centers on the geometric shape of an ellipse: a flattened circle or oval form.
橢 combines 木 (tree/wood) with 隋, which likely contributes the sound. The exact historical development of the ellipse meaning is uncertain.
The left side 木 suggests something wooden, while the right side 隋 looks like a rounded, drooping shape. Imagine a wooden oval frame or an elliptical wooden hoop.
For ダ, picture a wooden ellipse that is dark and dull: dark -> ダ, and the dark wooden ellipse stands out.
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