Kun stands in the north in the Earlier Heaven arrangement attributed to Fuxi, directly opposite Qian in the south, and the two together define the axis on which that whole symmetrical order is built. In the Later Heaven arrangement attributed to King Wen, Kun moves to the southwest, and Qian to the northwest, so the pure yin and pure yang trigrams that anchored the first diagram are pushed to adjacent corners of the second.
That displacement is the single clearest sign that the two arrangements are answering different questions. Earlier Heaven puts heaven and earth on the vertical axis because it is describing an ideal cosmos in balance. Later Heaven is describing the world as it turns, and in a diagram organised by the seasons the trigram of the receiving earth belongs to late summer, when what was planted is being carried and filled out rather than begun.
In the Later Heaven order Kun belongs to the phase Earth, shared with Gen. From the Shuogua it takes the ox among animals, for patient strength under a load, and the belly among the parts of the body, the part that contains and nourishes. In the family scheme Kun is the mother, the trigram of unmixed yin from which each of the three daughters takes its single yin line.
Kun and Qian are the two trigrams that have no mixed lines at all, which is why every other trigram in the system can be described as one of them with a line exchanged.
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