[河出图，洛出书，圣人则之]
Legend has it that the He-Tu Formation （河图）was derived by Fu Xi, who observed unique patterns of spots off the back of a mythical creature called a Dragon Horse （龙马） that emerged from the Yellow River （黄河）.
By use of numbers to represent the spot formations, in evaluating the arrangements of the He-Tu Formation;
- 1 and 6 is at the North, representing the Water element [一六共宗，属水，位居北方]
- 2 and 7 is at the South, representing the Fire Element [二七同道，属火，位居南方]
- 3 and 8 is at the East, representing the Wood Element [三八为朋，属木，位居东方]
- 4 and 9 is at the West, representing the Metal Element [四九作友，属金，位居西方]
- 5 and 9 is at the Centre, representing the Earth Element [五十同途，属图，位居中央]
Elemental Interaction
There is a Cycle of Birth relationship between the Five Elements
（五行）, where Water gives birth to Wood, Wood gives birth to Fire, Fire
gives birth to Earth, Earth gives birth to Metal, and Metal in turn
gives birth to Water to complete the Cycle of Birth. (Note: the
"Elements" should not be interpreted in the literal sense)
Numberology
孔子云：“ [天以一生水，而地六成之； 地以二生火，而天以七成之； 天以三生木， 而地以八成之； 地以四生金， 而天以九成之； 天以五成土， 而地以十成之。]
The odd numbers or 奇数 are Yang numbers and are also known as 天数, while the even numbers or 偶数 as Yin are also known as 地数. Within each of the combinations, there is a 天数 and a 地数, and hence there is balance of Yin and Yang.
The numbers 1, 2, 3, 4, and 5 are the base numbers and are known as 生数, while the numbers 6, 7, 8, 9, 10 are the resulting numbers and are known as 成数. Within each of the combinations, there is a 生数 and a 成数. We can see that within the combinations, the subtraction of the 生数 from the 成 数 will always equate to 5, which is in turn called the 小成.
From the perspective of adding the numbers across the cardinal directions and ignoring the centre, there is a combination of 20 （合二十）, i.e. 3+8+2+7 = 20, and 1+6+4+9 = 20. From a different perspective, adding up all the odd numbers (1+3+7+9) and adding up all the even numbers (2+4+6+8) will both yield 20 as well. (5 and 10 not taken into consideration here)
Applications
From the perspectives of Yin Yang and Elemental Interaction, we can see that the He-Tu depicts a state of harmony which is conducive to "birth". The combinations of the Heavenly Stems （天干）, which will be covered in a later posting, also follows the relationships of the He-Tu Formation.
The He-Tu Formation is closely related to the Early Heaven Ba Gua arrangement （先天八卦）, which has its applications in the San Yuan （三元） systems of Xuan Kong Da Gua Feng Shui （玄空大卦风水） and Xuan Kong Da Gua Date Selection （玄空大卦择日）.
Point to ponder - since Fire and Water, as well as Metal and Wood are opposite each other, wouldn't they clash? Or are they complementary and in harmony?
