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This particular pose was one of Boucher's favorite themes. It occurs repeatedly in his works, and art historians have not come up with any particularly persuasive reason for it. Obviously it a picture of a a beautiful young nude girl, but why she would willingly put herself into such an odd posture just to show off her naked rear-end is unclear.
Nevertheless, it becomes one of Boucher's lasting contributions to art, and the 19th century is full of Odalisques, few of them with such modesty.