The He Tu river map and the Luo Shu square are mentioned in Xi Ci Zhuan (the commentary on the text tagging) but without any illustration. The characteristics of the eight trigrams and the relationship between each two trigrams are described in Shuo Gua Zhuan (the commentary on the trigram) but without mentioning any diagram neither.
It is said that all these map, square and diagrams known to us were drawn later by Master Chen Tuan (陳摶, an archaeologist and the founder of Zi Wei Dou Shu (紫微斗數) in the 9th century) according to his study of the I Ching. From that he also derived the relationships among the five basic elements: metal, wood, water, fire and earth, which are used for most of Chinese fortune-telling methods.
The following deductions, showing how the post-heaven diagram is formed, are made according to the aspect of the five elements.
1) The He Tu river map and the five elements2 and 7 locate at the top of the river map; the top (of the ancient Chinese constellations) is designated as the south, and the south is where fire is attributed. 1 and 6, therefore, is in the north where water is; 4 and 9 is in the west where metal is; 3 and 8 is in the east where wood is; 5 locates in the centre where earth is..
Remarks: a) The ancient Chinese faced the Plough to observe celestial phenomena; in this way the south is on top (of the constellations) and the north is at the bottom. b) The attributes of the five elements mentioned above are designated according to the Chinese geography and climate: the south is warm, the north is cold, the west is the mining area and the east is farmlands.
The river map reveals the relationships among the five elements in the form of reproduction: fire reproduces earth; earth reproduces metal; metal reproduces water; water reproduces wood, and wood reproduces fire.
The one reproducing is the parents, while the one reproduced is the children. Therefore the pre-heaven diagram is seen as the origination of the eight trigrams.
2) The Luo Shu square and the five elementsThere is another relationship, subduing, among the five elements: metal subdues wood; wood subdues earth; earth subdues water; water subdues fire, and fire subdues metal.
The one subduing others is the commanding officer, while the one subdued is the property of the commanding officer; for instance, metal is the master of wood and it can carpenter woodenware to make a profit, and wood needs metal to make itself become a useful stuff.
Following the attributes and directions of the five elements obtained above, the Lou Shu square exhibits their subduing setup.
3) The forming of the post-heaven diagram
Kan is water, therefore, put in the north, and Li is fire and in the south. Qian and Dui are in the west of the river map; therefore they are attributed to metal and located in the northwest and the west, respectively. Zhen and Xun are attributed to wood; therefore they are located in the east and the southeast, respectively. Kun and Gen are earth; therefore they stay in the middle (i.e. at the diagonal positions which separate wood and fire from water and metal). Hereby the post-heaven diagram is formed.
The attributes and directions of the eight trigrams are displayed in the post-heaven diagram; therefore it is said that the post-heaven diagram relates to the application of the eight trigrams.
4) Integrating the post-heaven diagram with the performances and locations of the eight trigrams regulated by Shuo Gua Zhuan
a) Zhen and Xun are attributed to wood; wood locates in the east as the east is farmlands.
All creatures originate from Zhen (to move, the thunder), like the plant being aroused by the spring thunder starting to sprout. It locates in the east.
All plants grow in unison at Xun (to enter, the wind), like blooming sunflowers being bent widely over grasslands by the wind. It is in the southeast.
b) Li is fire and locates in the south as the south is warm.
Li (clinging, fire) provides brightness and makes all creatures visible and able to see one another. It is a trigram of the south.
c) Kun and Gen are earth; they locate in the middle (i.e. at the diagonal positions, which separates wood and fire from water and metal).
Kun (submissiveness, earth) is earth, whereby all creatures are nourished; therefore it is said: all being served by Kun. It locates in the southeast.
d) Qian and Dui are in the west; they are ascribed to metal.
Dui (joy, the marsh) is the autumnal equinox, where all creatures feel joyful while reaping crops and celebrating the fruitful harvest.
Battling at Qian (perseverance, heaven) means masculinity and femininity approaching each other at Qian. After the autumnal equinox, the bright masculine is overpowered by the shaded feminine. It is a trigram of the north.
e) Kan is water and locates in the north as the north is cold.
Kan (the abyss, water) is water. It is a trigram of the north. It is also a trigram of toil or fatigue as all life must work hard in the cold north and they return home where all creatures belong after almost one entire year working and getting tired. Therefore it is said: toil or fatigue at Kan.
f) Kun and Gen are earth; they locate in the middle (i.e. at the diagonal positions which separate wood and fire from water and metal).Gen (keeping still, the mountain) is a trigram of the northeast. All creatures have accomplished their tasks and (are going to) have a new beginning; therefore Gen is seen as accomplishment.