Also known as: Snake Branch
Si is yin fire, hidden stems 丙 (yang fire), 庚 (yang metal), 戊 (yang earth) — a fire that already carries metal and earth, ready to be tempered into precision. Symbolically the snake basking on a sunny stone, beautiful and watchful. Si Day Masters often have refined intelligence, eloquence, and a strategic patience that strikes only at the chosen moment.
Si san-he with 酉 and 丑 transforms to metal (巳酉丑合化金), and six-unites with 申 to water (巳申合水, also a punish/half-clash relation). Conflicted by 亥 (巳亥冲), and part of the 寅巳申 三刑.
The most intriguing thing about Si (巳 sì) is that a fire branch houses the birth of metal. The twelve-stage life cycle places the birth stage of Geng metal in Si, and Si is fire; fire controls metal, so setting metal's beginning in the thing that controls it looks backwards. The classical explanation takes the image of forging: metal becomes an implement only through fire, and ore entering the furnace is where it truly starts becoming metal. Once that is granted, the Si-You-Chou harmony producing metal follows naturally — the furnace lights in Si, peaks in You, is stored in Chou.
Follow that life-cycle line a little further and it runs into the largest disagreement in the four-pillar tradition: where yin stems begin. One camp holds that yin and yang are born and die together, so Yi shares Hai as its birth position with Jia and treats Si as a place of exhaustion, since one family should not run two rule sets. The other holds that yang runs forward and yin runs backward, giving Yi its birth in Wu, Ding and Ji in You, Xin in Zi, Gui in Mao. Later compendia record both views side by side. At Si the two produce opposite conclusions, so anyone arguing from this position should say which system they are using.
The Si month opens at Start of Summer and closes before Grain in Ear, with Grain Full as its mid-term: grain filling out but not yet ripe, heat arrived but not yet severe. Bing fire reaches its office stage in Si and Wu earth takes its prosperity position there too, so fire and earth flourish together all month. Reading a chart of this month, the tradition looks first for water: fire and earth scorched with no water is called burnt out, while water and fire in mutual completion is where distinction is thought to arise. Water in summer and fire in winter are the basic pair of the climate method.
Si belongs with Yin, Shen and Hai to the four growth positions, which the symbolic-star tradition calls the travelling horse; for the Hai-Mao-Wei group the horse sits in Si. These four are about beginnings and movement, and combined with the fire nature of Si the tradition describes a mind that starts quickly and a temperament unwilling to sit still. Directionally Si shares the southeast with Chen under the Xun trigram, and Xun is wind and entering, an image that echoes the travelling theme. Still, symbolic stars are a side channel; whether this Si is wealth, authority or output for the day master is the main line.
The Si and Shen pair shows best why branch relations cannot be memorised as slogans. Si and Shen are in six union, in punishment, and in harm — three relations stacked on the same two characters. The classical handling is to look at conditions. Close together with water present to draw the transformation, the union dominates. In a chart already heavy with metal and water and then struck, the punishment dominates. If each is already occupied by another branch, they may barely engage. Branch relations are conditional statements rather than equations, and a lookup table only supplies candidates.
Demoting Si to a small fire misreads the word yin. In yin fire it refers to a contained form, not to weak force: Si is the office position of Bing fire and its actual heat does not fall short of Wu, it simply does not broadcast the way Wu does. The second reading carries the snake image directly into character reading and comes out with something secretive or scheming. Classical material leans the other way, toward brightness and quickness of perception, and the hours Si governs are nine to eleven in the morning, the sunniest, most open stretch of the day.
The Si double hour runs from 09:00 to 11:00, late morning with the sun climbing toward its height. As a month Si is the fourth month of the solar-term year, opening at Start of Summer and closing before Grain in Ear, with Grain Full as its mid-term, an image of heat that has arrived and is still rising. In the solar-term calendar Start of Summer does open the Si month and the summer quarter, and BaZi changes the month pillar there. In terms of actual weather it is earlier than the hottest period, which comes around Minor and Major Heat two months later. The solar terms mark stages of the sun's cycle, so the season's name describes where the cycle stands and not what the thermometer reads.
Because the twelve-stage cycle reads birth as the start of a process, not as comfort. The classical image is forging: ore only becomes an implement by entering fire, so the furnace is where metal's career begins. That is also why the metal-producing harmony runs Si to You to Chou, from kindling to peak to storage. It is a statement about process, not a claim that fire nourishes metal in the ordinary productive sense.
Both are recorded, along with harm, and which one acts depends on the chart. When the two sit adjacent and water is present to draw out the transformation, the six union producing water is emphasised. When the chart is struck from outside or already heavy with metal and water, the punishment reading is emphasised. Classical practice treats stacked relations as conditional, resolving them by season, closeness and what else each branch is already engaged with.
Wu is yang fire at the peak of summer, open, radiating and obvious, holding Ding fire with Ji earth. Si is fire at its office stage, contained in form, and it holds Geng metal and Wu earth as well as Bing fire, which makes it fire with material already in it. Wu tends to be read as display and intensity, Si as precision and calculation. Si is not the weaker of the two, only the less demonstrative.