This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Fire Branches gathers the two of the twelve Earthly Branches whose own fixed agent is Fire: Si the Snake and Wu the Horse, the branches that open and then crown the summer.
Si, the sixth branch, opens the fourth solar month at the Start of Summer and governs the hour from nine to eleven in the morning, in the south-southeast. It is a growth branch, storing Bing as its principal qi alongside Wu and, notably, Geng, so Si carries the birth of Metal concealed inside its own rising Fire; as the branch that opens the Metal trine, it is the point at which the coming season's harvest is already seeded within summer's own heat. Wu, the seventh branch, sits at the summer solstice and governs the hour from eleven to one, seated due south. It is a cardinal branch, storing Ding as its principal qi alongside a concealed Ji, so its blazing yang surface holds a yin heart within it, and as the cardinal Fire it anchors the Fire trine it forms together with Yin and Xu. Wu marks the year's highest point of yang, the hour after which yin begins its slow return.
Between them the two Fire branches trace summer's whole arc, from the first rising heat Si carries to the peak Wu marks and past which the year turns. Like every branch, both are day-count signs first, from the oracle-bone tradition that combined the twelve branches with the ten stems into a sixty-day cycle before any animal was attached to either of them.