foothill
岊 centers on a low hill at the base of a mountain: a foothill.
岊 combines 山 (mountain) with 巴, which likely contributes a phonetic element. The exact historical development is uncertain.
The top component 巴 looks like a curled shape, and 山 is a mountain. Picture a curled, low rise at the foot of a mountain: a foothill.
For セツ, imagine a set of foothills at the base of a mountain range: set -> セツ.
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