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flat; even; level
plain (language, words, etc.); simple
川辺で拾った平たい石を水面に投げて水切りをして遊んだ。
We played by throwing flat stones into the river.
難しい話を平たく説明してください。
Please explain the difficult matter in plain terms.
この石は表面が驚くほど平たかった。
The surface of this stone was surprisingly flat.