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The human body has twelve primary meridians. Zhang Zhongjing established only the Six Meridians (Liu Jing) differentiation system in *Shanghan Lun* (Treatise on Cold Damage), based on the laws of the Six Qi (Wind, Cold, Summer-heat, Dampness, Dryness, Fire). For the Shaoyin (Lesser Yin), Shaoyang (Lesser Yang), and Yangming (Greater Yang) meridians, the Hand meridians govern the Qi transformation, while the Foot meridians follow to transform accordingly. For the Jueyin (Terminal Yin), Taiyin (Greater Yin), and Taiyang (Greater Yang) meridians, the Foot meridians govern the Qi transformation, while the Hand meridians follow to transform accordingly.

In the Six Meridians of *Shanghan Lun*, only the Foot meridians are discussed, not the Hand meridians, because the Foot meridians circulate throughout the entire body and their domains are extensive, whereas the Hand meridians circulate only in the two arms and their domains are small. In reality, both sets of meridians share the same Qi; when disease occurs, both are affected. Therefore, the discussion focuses on the extensive Foot meridians to generalize the small Hand meridians. It is not that only the Foot meridians become diseased while the Hand meridans remain unaffected. Whenever symptoms such as cold limbs, reversal (Jue), pain, or numbness are mentioned, the Hand meridians are also included and do indeed become diseased.

The Foot-Taiyang Bladder Meridian (Taiyang) governs with Cold-Water Qi, and the Hand-Taiyang Small Intestine Meridian's Fire Qi transforms into Cold Qi accordingly. The Hand-Yangming Large Intestine Meridian governs with Dry-Metal Qi, and the Foot-Yangming Stomach Meridian's Earth Qi transforms into Dry Qi accordingly. The Hand-Shaoyang Sanjiao (Triple Burner) Meridian governs with Minister-Fire Qi, and the Foot-Shaoyang Gallbladder Meridian's Wood Qi transforms into Fire Qi accordingly. The Foot-Taiyin Spleen Meridian governs with Damp-Earth Qi, and the Hand-Taiyin Lung Meridian's Metal Qi transforms into Damp Qi accordingly. The Hand-Shaoyin Heart Meridian governs with Sovereign-Fire Qi, and the Foot-Shaoyin Kidney Meridian's Water Qi transforms into Fire Qi accordingly. The Foot-Jueyin Liver Meridian governs with Wind-Wood Qi, and the Hand-Jueyin Pericardium Meridian's Fire Qi transforms into Wind Qi accordingly. This is the normal physiological state of the Six Meridians.

When disease occurs, Taiyang manifests as Cold, Yangming as Dryness, Shaoyang as Fire, Taiyin as Dampness, and Jueyin as Wind. However, Shaoyin does not transform from Heat but rather from Cold. Because Fire prevailing causes Water to retreat, and Water prevailing causes Cold to flourish. In disease, Water can overcome Fire, but Fire cannot overcome Water. Therefore, Shaoyin transforms into Cold following Ren-Water (Yang Water/Cold Water), rather than transforming into Heat following Ding-Fire (Yin Fire/Sovereign Fire).

Regarding the Yangming meridian, when Yang Qi is abundant, it follows Geng-Metal (Yang Metal) to transform into Dryness; when Yin Qi is abundant, it follows Ji-Earth (Yin Earth) to transform into Dampness. It is not always excessive Dryness. (The upper volume of *Shanghan Lun* discusses Yangming diseases characterized by excessive Dryness, while the lower volume discusses those characterized by excessive Dampness). Regarding the Shaoyang meridian, when Yang Qi is abundant, Fire is vigorous and transmits to the Fu organs; when Yang Qi is deficient, Fire is weak and transmits to the Zang organs. It is not always excessive Fire.