One would think that mid-19th Century knee pants were a natural evolution from 18th Century knee breeches.

有人认为19世纪中期的短裤式样是承自18世纪的自然演变。

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Pleats were big in menswear until the mid-'50s, when flat-front pants arrived with that era's slim Continental look, and stayed in vogue, only ending when boxy looks took over in the '80s.

WSJ: The Pleats Plea: Wear These