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Zhang Sanxi stated: The *Jin Jing Lu* (Golden Mirror Record) documents thirty-six tongue presentations for differentiating the depth and prognosis of Shanghan (Cold Damage), which can be considered detailed and comprehensive. However, upon careful investigation, these ultimately reduce to the Eight Principles (Ba Gang): Yin, Yang, Exterior, Interior, Cold, Heat, Deficiency, and Excess.

Tao Jie'an stated: When pathogenic factors of Shanghan are in the Exterior, there is no coating on the tongue. If heat pathogens are present [note: text implies progression], the coating gradually forms, changing from white to yellow, then to black, and even cracking with a black appearance. Black coatings often indicate a poor prognosis. If the root, center, or tip of the tongue is black, it belongs to Interior Heat syndromes. A completely black tongue indicates extreme heat and is difficult to treat. It is commonly seen that external contraction complicated by internal injury and retained food stagnation, severe and knotted below the heart, results in the tongue gradually turning yellow after five or six days, or becoming dry in the center while moist at the edges; this is called "Zhong Bei Tongue" (Middle Roasted Tongue). In such cases, the Interior Heat is still shallow. If the coating is completely dry, regardless of whether it is yellow or black, it belongs to an Interior syndrome, and purgative methods should be used according to severity. If purgatives have been used previously or repeatedly without reducing the coating, it indicates that old stagnation is knotted in the Middle Jiao (Spleen and Stomach). One must diagnose the deficiency or excess of the pulse and the strength of the Middle Qi. For excess patterns with a moist surface, purgatives may be used; for deficient individuals with insufficient Shen Qi (Spirit Vitality), one should generate fluids and consolidate the Middle Qi. Some have been cured using Sheng Jin San (Fluid-Generating Powder) combined with Jie Du Tang (Detoxifying Decoction); others have been cured using Fuzi Lizhong Tang (Aconite Regulate the Middle Decoction) taken cold. The former represents Extreme Yin resembling Yang, and the latter represents Extreme Yang resembling Yin; these must not fail to be distinguished.

White coating indicates Cold. If external symptoms include irritability and a desire to sit in mud or water, this is due to Yin-Cold forcing the rootless, unanchored Fire outward. If the pulse is large but does not drum against the fingers, it should be treated as a Yin syndrome. If there is no extreme restlessness but vomiting occurs, it should be treated as a Food-Stagnation Yin syndrome.

Dry White Coating: Due to deficiency with slight heat, or inability to sweat, or slight retention of fluids in the Stomach not moving, warm resolution is appropriate.

Slippery White Coating: Due to deficiency-cold, Stomach cold, and failure of Yang Qi to flourish, warming is appropriate.

White Coating with Prickles: Due to insufficiency of body fluids and presence of material in the Stomach, movement (digestion/promotion) is appropriate.

Yellow Coating: Slight heat, heat gradually entering the Interior, or signs of dryness and thirst, clear resolution is appropriate.

Gray Coating: Presence of material in the Stomach, deficiency of Middle Qi, heat and thirst but inability to digest drinks, warm resolution is appropriate.

Black Coating: Heat entering the Interior, excess and dryness with thick accumulation, sliding down with lubrication is appropriate; if moist, it is water trapping fire, warming is appropriate. Although black and moist, this is described as "Extreme Water resembling Fire," differing from dryness.

In general, for Shanghan, tongue diagnosis is used because the tongue belongs to the Heart, which governs Fire, while Cold belongs to Water. Water-Cold overwhelms Heart-Fire, causing the tongue to suffer. For postpartum conditions, tongue diagnosis is used because the Heart governs Blood. The Classics state: "When Shaoyin Qi is exhausted, blood does not flow." Therefore, a purple-black tongue indicates that the blood has perished first.

Whenever a black tongue is seen, one must ask if the patient has consumed sour, sweet, or salty substances that could stain the tongue black. Whenever observing tongue color, even if one has preconceived notions, one must carefully examine accompanying symptoms and the deficiency/excess of the pulse; otherwise, a discrepancy of a hair's breadth may lead to an error of a thousand miles.