More on HeTu,
every 180o turn of the ground plane (地), complete two independent cycles (bottom arrows swap places resulting in 7 to 6 and 8 to 9),
6 1 2 7 turn 6... and 9 4 3 8 turn 9...
If we use binary to assign Gua,
艮 坤 震 离... and 乾 兑 坎 巽...
The cycle,
艮 坤 震 离 艮 坤 震 离 ...
is call DiDao (地道) containing DiGua (地卦) and
乾 兑 坎 巽 乾 兑 坎 巽...
is called TianDao (天道) containing TianGua (天卦).
From the diagram when we group Gua on the same plane, we have the pair 1-3, 2-4, 6-8, 7-9, or 坤-坎, 震-兑, 艮-巽, 离-乾.
Left side DiGua, right side TianGua and Gua on the same plane, alternate Gua around the periphery. Around the periphery, 乾 兑 坎 巽 坤 艮 离 震. This order has no particular meaning for the Gua progresses through the red loop, repeating as the ground rotates 180o.
The Gua on LuoShu (洛书) is a dynamic changing Gua that cycle between heaven and earth and so is the Ren Gua (人卦).
We stack the three trigrams Tian Ren Di and form a new Nonagram that is interpreted as two up-down overlapping hexagrams.
The problem with this arrangement is, sections of the original hexagram spread are not being used because the top and bottom Gua are both restricted to one in two distinct sets of four. If it is possible to invert this nonagram, all possible hexagrams can be represented.
Or, we generate two new hexagrams using TianGua and DiGua as the top Gua and, RenGua as the bottom Gua. Then all in the array of hexagrams are represented.
What is the logic in stacking up two trigrams into a hexagram in the first place?
