Zhen sits in the northeast in the Earlier Heaven arrangement attributed to Fuxi, opposite Xun in the southwest. In the Later Heaven arrangement attributed to King Wen it takes the east, and the east is where the applied traditions expect to find it.
Later Heaven is ordered by the cycle of the year, and the east is the spring and the sunrise, the quarter where things begin. A trigram whose attribute is thunder, the shock that starts movement, belongs at exactly that point, and the Shuogua says as much when it opens its account of the cycle with the god coming forth in Zhen. This is one of the places where the Later Heaven order is not merely conventional but argued for in the text itself.
In the Later Heaven order Zhen belongs to the phase Wood, which it shares with Xun, and to early spring. From the Shuogua it takes the dragon among animals, a creature associated with thunder and with the spring rains, and the foot among the parts of the body, since the foot is what moves first. In the family scheme Zhen is the eldest son, having taken the first yang line at the bottom.
Because Zhen and Xun share a phase and stand adjacent in the Later Heaven order while facing each other across the Earlier Heaven one, they are a good pair to test any diagram against when the arrangement is not labelled.
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