The design of twin-bi joint with a movable hinge dates back to the late Eastern Zhou dynasty. See an example in the Winthrop Collection dated to the late Eastern Zhou period, illustrated in Max Loehr, Ancient Chinese Jades, Fogg Art Museum, Harvard University, 1975, p. 342, no. 507. The Winthrop twin-bi is decorated with spirals and two protruding animal-shaped scrolls on the outer edges. A different prototype of smaller size, dating back to the Han dynasty, is from the Qing court collection and is preserved in the National Palace Museum, Taipei (inventory number: Gu yu 003335). It is carved from white and russet jade with chilong and dragons among clouds. The Han dynasty object has a square hinge ornamented with a mythical mask on one side and the characters yannian (‘longevity’) on the other.
While the archaic jades in the imperial collection might have served as inspirations for the Qing hinged-bi examples, the central turnable plaques appear to be a Qing dynasty innovation, with the yin-yang sign representing cosmology and the fu symbol signifying the righteousness of the emperor. The reverse of the hinge is decorated with raised dots reproducing a rare diagram of luoshu, also known as jiugongtu (‘nine-palace diagram’), jiugonsuan (‘nine-palace calculation’), also known as the “magic square” in the West. The vertical, horizontal and diagonal sums of a magic square are all the same; in this case, they all equal 15. Dating back as far as the Western Han dynasty, luoshu was believed to be bestowed by heaven to a just ruler only and to proclaim his legitimate power. The magic square on the hinge hence serves as a perfect bridge between the cosmos and the virtuous emperor.
Other recorded examples from the Qing dynasty of this form include a slightly larger twin-bi in the Palace Museum, Beijing, illustrated in Zhongguo yuqi quanji [The complete collection of Chinese jades], vol. 6: Qing dynasty, Shijiazhuang, 1993, pls. 250-251. One side of the discs is similarly carved with raised bosses, but the reverse is decorated with a dragon and a phoenix amidst flowing clouds, completing the auspicious symbolism of longfeng hebi (‘Complement of dragon and phoenix’). The embedded central turnable plaques are similarly reticulated with yin-yang and fu symbols surrounded by scrolling clouds. The angular loops are also modelled in the form of a pair of kui dragons, yet in a slightly less simplified form when compared to the current piece. Another twin-bi, also preserved in the Palace Museum, Beijing, is illustrated in The Complete Collection of Treasures of the Palace Museum. Jadeware (III), Hong Kong, 1995, pl. 128. It is from the Qing Court collection and is the largest among the three. One side is adorned with a dragon and phoenix among clouds, while the reverse is densely decorated with stylised scrolls. According to the catalogue entry, the discs are linked together by a square hinge similarly detailed in low relief with a mythical animal mask and raised bosses, possibly luoshu. See also a white jade example in the collection of His Majesty the King at Windsor Castle, included in the International Exhibition of Chinese Art, Royal Academy of Arts, London, 1935-36, no. 2327.
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