Level XVII of XX on the frequency path
Frequency rank 2216 / 2,608
Rarer in this ranking, but still part of the 2,608-character published set.
More common than 升 (Grid), less common than 蔽 (Cover).
The concave character looks like a depression pressed into a surface — the outline shows walls dipping down into a hollow center, visually sinking inward.
凹 is rank 2216 of 2,608 in published Japanese.
Study 凹 →