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height; stature
「背」 has another reading with different meanings.
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弟おとうとは私わたくしより背せが高いたかい。
My younger brother is taller than me.
背せが低いひくいから上うえの棚たなに手てが届かとどかない。
I'm short so I can't reach the top shelf.
最近さいきんまた背せが伸びのびたんじゃないかな。
I think you've grown taller again recently.