Dui is one of the eight trigrams (bagua) of the Yijing. It is composed of two yang lines below a single broken yin line, and its attribute is the Lake or Marsh, the Joyous, associated with pleasure, openness, and gentle expression. In the family symbolism it is the Youngest Daughter, and its polarity is yin. Its associated element is Metal, and in the Later Heaven arrangement of the bagua its direction is west. Each of the eight trigrams pairs and combines to form the sixty-four hexagrams through which the Yijing is consulted, the broken and unbroken lines shifting to give the oracle its answers. These are the conventions of the Chinese cosmological and divinatory tradition, in which Dui, the still and reflective water of the lake, expresses the quality of joy.
Dui's compass position is not fixed across the tradition: the Later Heaven direction given above is King Wen's arrangement, used in the practical, compass based feng shui of the Luopan; the older Fu Xi or Earlier Heaven arrangement, an idealised order rather than a worldly one, places Dui in the southeast instead, opposite Gen. Both arrangements assign Dui the family attributes and element given above; what changes between them is only the trigram's position on the circle, Later Heaven describing the world of change and manifestation the Yijing addresses, Earlier Heaven a prior, static order that Chinese cosmology treats as underlying it.
Sources Fathoming the Cosmos and Ordering the World: The Yijing (I Ching, or Classic of Changes) and Its Evolution in ChinaRichard J. Smith with The I Ching, or Book of ChangesRichard Wilhelm (trans.); Cary F. Baynes (English trans.), The Five "Confucian" ClassicsMichael Nylan