The secret of SanHe 三合 and SanYuan 三元 LuoPan are within the writing of TianYu Jing 天玉經 - a single classic that can be interpreted according to both schools. TianYu Jing itself has a number of versions. But for the purpose of discussion we shall inspect the YangGong Old Method's version and DiLi BianZhen's version. Listed herewith is the YangGong Old Method's version proudly brought to you by the late Grand Master Li SanSu, uncle to the late Grand Master Li DingXin. This version of the classic can be sub divided into the following grouping, for the ease of understanding: -
TianYu Jing 天玉經 - Dragon Water Convergence 龙水交会 is relevant to our topic of discussion - YinYang 陰陽 of the 24 mountains 二十四山 of the SanYuan LuoPan. There are 24 mountains. In fact, there are only 12 pairs of YinYang Dragons 十二陰陽龍. 12 YinYang all the way has within the incoming Gua 十二阴阳一路排，总是卦中来. These 12 pairs must be aligned to form Dragon, Mountain, Facing and Water 龍山向水 (DMFW). The classics state, Topsy turvy Parents to array Dragon position, Mountain facing and corresponding Water, 倒排父母荫龙位，山向同流水. This is the structure of SanYuan (or SanHe?) in Qi connections 三元氣圖.
Structure aside, there are only two ways to spin your YinYang. The classics state, Divide establish YinYang 2 different ways, 分定阴阳两路行. The left turning is Yang, the right turning is Yin. The classics state, left turning and right turning to arrange them, 顺逆推排定. However, left or right would only make sense from a fixed point of observation, the datum (天心). This datum is then referred to as the west or the east side of the river bank, resulting in the East River and West River sides. The classics state, divide into East and West two Gua, those who understood will announce to the world, 分却东西两个卦，会者传天下. These are the 2 different Gua – Grouping? In summary, the 12 pairs of YinYang Dragon is to identify left (Yang) or right (Yin) turning. Observe how your Qi flow is the foundation of YinYang. Others are just - crap.
TianYu Jing also identify that the Yang Stems are all left turning - Yang (these are called Dragon God 龍神. The classics state, Jia, Geng, Bing, Ren, are Yang, left turning to find the 5 element structure, 甲庚丙壬俱属阳，顺推五行详. Dragon God 龍神 can be construed as an entire independent topic by itself... however, TianYu Jing - Establishing Facing 定向 has this to say, Front twice yielding Dragon God front twice yielding Facing, pearl string do not face to release. Back twice yielding Dragon God back twice yielding Facing, Arraying established YinYang counts, 前兼龙神前兼向，联珠莫相放。后兼龙神后兼向，排定阴阳算. In other word, Leaning Star polarity depends on the Dragon God 龍神.
The Yin Stems are all right turning - Yin (these are called Water Dragon). The classics state, Yi, Xing, Ding, Gui are Yin, right turning to find the 5 element structure, 乙辛丁癸俱属阴，逆推论五行. The classic did not, however, identify explicitly the rest of the 16 dragons - the 4 ordinal Gua, 4 growth branches, 4 stable branches and 4 graves branches.
The four Ordinal Gua (surprisingly that YangGong referred to the Later Heaven BaGua) were paired respectively as the followings – GenYin 艮寅, XunSi 巽巳, KunShen 坤申, QianHai due to the double mountain and because Yin Si Shen and Hai 寅巳申亥 are the four growths of the Dragon God 龍神, they take Yang 陽. In the method of Pushing Sha and Receiving Water, Jia Geng Bing Ren four Dragon God, entirely belongs to Yang stem element. Qian Xun Gen Kun water facing the same, nobility with access prosperity, 甲庚丙壬四龙神，俱属阳干行。乾巽艮坤水向同，富贵足兴隆. Yin Shen Si Hai water incoming without broken, 5 element facing within storage, 寅申已亥水来长，五行向中藏.
The four Graves were paired in the similar double mountain fashioned such as GuiChou 癸丑, YiChen 乙辰, DingWei 丁未, XinXu 辛戌 with the respective 4 Yin Stems and therefore they are Yin 陰. The four Stables were implied as Yin 陰. The classics state, Chen Xu Chou Wei one way move, sudden prosperity shake the family, 辰戌丑未一路行，骤富振家声. Chen Xu Chou Wei knock against Golden Dragon, Wealth and prosperity never cease to expire, 辰戌丑来叩金龙，富贵永不穷.
Therefore the YinYang grouping of the 24 Mountains, taking reference from the 4 Cardinal Gua and the 4 Ordinal Gua, the different grouping of the 24 Mountains is as follows: -