Every stem has a partner among the other nine, and the five pairings, called the combinations of the heavenly stems, are fixed: each stem combines with the one five places away from it. Jia pairs with Ji, and the tradition holds that a combined pair takes on a phase of its own distinct from either stem's native phase. The pairing is used in almanac work and in fate calculation, where a combination in a chart is read as a binding between the two positions that carry it.
The ten stems are older than the sexagenary cycle they now form half of. On the Shang oracle bones of the second millennium BCE they name the ten days of the xun, the Shang week, and Shang kings are known to history by posthumous names built on them, so a stem was a unit of the working calendar before it was a term of cosmological art. The five-phase and yin-yang readings attached to the stems are later, systematised under the Han, and the atlas records the two layers as two layers.
In the Four Pillars of Destiny, the fate-calculation system built on the stems and branches, a stem is read as a day master when it heads the day pillar, and Jia as day master is treated as yang Wood in its most upright form, the standing tree rather than the climbing vine that Yi represents. The distinction between the yang and yin members of a phase pair is one of manner and not of substance.
The stem's own attributes here are Wood and yang polarity. Those two facts fix its place in the run of ten, since the stems proceed through the five phases in generation order taking the yang member first and the yin member second, and they fix how it is read wherever it appears, in the year, the month, the day or the double hour of a Four Pillars chart.
Sources Chinese Mathematical Astrology: Reaching Out to the StarsHo Peng Yoke with The Chinese Sexagenary Cycle and the Ritual Origins of the CalendarAdam Smith