smooth, hard spot made level for sacrificial altar
墠 centers on a leveled, smooth earthen surface, especially one prepared for ritual use as a sacrificial altar.
墠 combines 土 (earth) with 單, which likely contributes the sound and may relate to the idea of a single, leveled area. The exact historical development of 單 in this context is uncertain.
The left side is earth 土, and the right side 單 looks like a grid of fields. Picture a patch of earth smoothed flat like a prepared altar.
For セン, imagine a smooth earthen altar where a ceremony sends (セン) smoke upward: send -> セン, and the leveled ground makes the ritual possible.
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