Xiāntiān and hòutiān 先天•後天 before Heaven and after Heaven, prior to Heaven and posterior to Heaven, pre-celestial and post-celestial
““Before Heaven” refers to the stage before pre-cosmic Yin and Yang join together, while “after Heaven” is the stage after they join and generate the cosmic pneuma. A notable aspect of this process is that original, pre-cosmic Yin and Yang are each enclosed within their opposites in the cosmos. This notion is referred to by the phrases “Yin within Yang” (yáng zhōng zhī yīn 陽中之陰) and “Yang within Yin” (yīn zhōng zhī yáng 陰中之陽).” —Fabrizio Pregadio.
“A similar representation of cosmogony is seen in the xiantian and houtian arrangements of the eight trigrams (bagua), the first of which is traditionally attributed to the legendary emperor Fú Xī 伏羲 and the second to King Wen of the Zhou (Wénwáng 文王, r. 1099-1050 BCE). The xiantian diagram reproduces the stage after Original Yin and Yang (kun at due North and qian at due South) have joined their essences and have generated the trigrams li and kan at due East and West; the other four trigrams are placed at the intermediate points. Here the cosmos is generated after li and kan attract each other and join their inner lines. In the houtian arrangement, the positions originally occupied by qian and kun are taken by li and kan, to show that the shift from the unconditioned to the conditioned state has occurred, and that Original Yin is now found within cosmic Yang (the trigram li or Fire), and Original Yang within cosmic Yin (the trigram kan or Water).”—Fabrizio Pregadio.
The entry in the encyclopedia was written by its editor, F. Pregadio but the fist time I had seen this theoretical statement about xiāntiān and hòutiān relationships was on the graphical page in Bagua quanxue by Sūn Lùtáng 孫祿堂 (1860-1933), 1990 (actually it was the Russian translation of Chinese publication of 1917, Beijing): Zhang 19 Bāguà xiānhòutiān héyītú 八卦 先後天合一圖.
The Fú Xī 伏羲 sequence —xiāntiān 先天—of trigram (the same for I Ching) is Qián Dùi Lí Zhèn Xùn Kăn Gèn Kūn (from left to right and from down to up) and here it is the inner circle in the picture which begins on the South (Qián up) and goes counterclockwise to Zhèn, jumps diagonally to South-West and goes clockwise to Kūn, North (down).
The Wénwáng 文王 sequence —hòutiān 後天—of trigram is outer circle and once in the text it goes Qián Kūn Zhèn Kăn Gèn Xùn Lí Dùi and in the picture it jumps every time with Lí and Kăn (representing South-North) and Zhèn Dùi (representing East-West). The Wénwáng 文王 sequence underlines Father-Mother (Qián Kūn), three sons and three daughters relationships in the circle of family. The next Zhang 20 in the book of Sun Lutang will explain how martial artists interpret cosmogony for their use in everyday life.
In the beginning there is a scan with glyphs of one graphical page from the Bāguà Quánxué 八卦拳學 book and I placed it here because other pictures from online search very often are just not the same as the author Sūn Lùtáng 孫祿堂 used in his book and teaching. The last picture has only two sequences of trigram, xiāntiān and hòutiān (先天•後天) before Heaven and after Heaven.