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在 supplies the sound — zài shifting to chá, a z/ch alternation common in the same sibilant group. The 'at, exist' meaning of
在 plays no role here; pure phonetic. The stubble sense persists in
茬口 (chákǒu) 'crop rotation', and the metaphorical
茬 'a batch, an occasion' lives on in
那一茬人 'that generation of people'.
