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The classic texts state: "If one can combine the observation of complexion and pulse, treatment can be perfect." The tongue coating is the most easily observable aspect of the complexion (Qi color). In ancient times, Zhang Zhongjing wrote *Shanghan Lun* (Treatise on Cold Damage) and *Jingui Yaolue* (Essential Prescriptions of the Golden Cabinet), both detailing tongue coatings. Later works such as *Shanghan Shejian* (Observations on Tongue in Cold Damage) and *Maili Zhengyi* (Correct Meaning of Pulse Principles) were all developed based on these foundations. Among them, Du Qingbi's *Yanzheng Shefa* (Verification Methods for Tongue Diagnosis) is particularly concise. This is because the tongue is the orifice of the Heart; diseases of the Zang-Fu organs must manifest on the tongue. Eye diseases originate from the Zang-Fu organs, so it is appropriate to differentiate them by combining tongue coating analysis. This method applies mainly to external contraction diseases, while internal injury diseases rarely present with such signs.

Generally, at the onset of symptoms, if the tongue coating is white, thin, and pale, it indicates that cold pathogen has just entered the Taiyang channel. If white flakes cover the entire tongue, the pathogen is about to transmit to the Yangming channel. A white coating that is dry and thick indicates a heat disease of the Taiyang channel. A white coating that is slippery indicates a concurrent disease of Taiyang and Yangming. A white coating with slight yellow in the center indicates that the pathogen of Taiyang is gradually entering Yangming. If the exterior syndrome is not resolved, use a dual-release method; if the exterior is resolved, use purgation. If diarrhea occurs, use clearing and draining methods. A white coating with slight prickles at the tip indicates a concurrent disease of Shaoyang and Yangming. A white coating with black spots in the center also indicates Shaoyang and Yangming involvement. If the exterior is not resolved, use harmonization; if resolved, use mild purgation. A white coating fully covered with black prickles indicates a combined disease of the three Yang channels. White coating on the left indicates deficiency-heat in Yangming; white coating on the right indicates pathogen affecting Shaoyang. A white coating that is greasy and slippery indicates phlegm.

A tongue without white coating, accompanied by external symptoms of cold limbs, indicates cold directly invading the Shaoyin channel. If the white coating is on the left or right, while the rest shows yellow and black, accompanied by external symptoms of dysentery and pain radiating to the lower abdomen, this is Zang-Jie (organ knotting). For severe heat, use purgation; for no heat, use warming. White coating with a red tip indicates Shaoyang pathology. Tongue coating white at the root and red at the tip indicates concurrent disease of Taiyang and Shaoyang. A white tongue without coating, clear and pale, with external fever, indicates Stomach Qi deficiency. Any change in tongue color without coating belongs to deficiency. A white coating thick like accumulated powder indicates plague pathogen initially attacking the Membrane Source (Mo Yuan). A white coating turning black at both tip and root indicates excessive Metal and Water, with Fire and Earth Qi exhausted internally; although no ominous signs appear externally, death is inevitable. Two black lines in a white coating indicate pathogen from Taiyang and Shaoyang entering the Stomach due to exhaustion of Earth Qi, causing chest binding and pain. Two gray lines in a white coating indicate tongue changes from consuming cold food.

A slightly yellow coating indicates pathogen entering Yangming, representing interior heat syndrome. Deep yellow appearing subsequently indicates increasing heat. Dry or scorched yellow appearing later indicates extreme heat, requiring purgation. Yellow coating at the root and white at the tip indicates more interior than exterior syndrome. Slippery yellow coating indicates Damp-Heat in Yangming. Yellow coating with partitions on top indicates deep pathogenic toxin, requiring urgent purgation. Double-hanging yellow coating indicates Zheng-Yang Yangming syndrome. Yellow coating with macules indicates impending eruption of rashes; if no macules, use specific purgation. Prickles in yellow coating indicate severe Stomach heat. Small black dots in yellow coating indicate pathogen entering the Zang, requiring urgent purgation. Long-standing yellow coating turning black, dry and prickle-covered, indicates extreme excess heat, often untreatable. In such cases, scrape open the coating; if the base under the partition is red, urgent purgation is possible; if black, it is untreatable. Thick, dry black coating in the center indicates heat excess and fluid depletion. Black tongue without coating but dry indicates injured fluids and deficient fire acting. Black tongue with moisture and red edges, presenting with delirium, should be treated with simultaneous attack and tonification for the weak, and urgent purgation for the strong. Summer stroke often presents this tongue; treat with acrid-cool release. In cases of extreme deficiency, combine pulse and symptom analysis. Black center with white slippery edges indicates exterior and interior deficiency-cold.

Pure black coating presents two syndromes: one is extreme fire resembling water, the other is water overcoming fire. Differentiation: fire excess shows thick coating with many prickles; water overcoming fire shows thin coating without prickles. Black tongue with central ulceration is untreatable. Dry, black, short tongue is untreatable. Purple-black tongue postpartum is untreatable. When seeing black, ask if sour, sweet, or salty foods were consumed, as they can dye the tongue black; such tongues are moist, not dry, and the color wipes off. Gray coating is a milder form of black, treated similarly, with Yin-Yang differentiation. Direct invasion of Yin channels causes immediate gray-black tongue without accumulation; transmitted heat syndromes cause dry gray-black coating. Generally, yellow, white, and black have coating, while red and purple show color without coating. Pure red tongue indicates extreme fire excess and deepening plague. Red center indicates Taiyang syndrome. Red tongue with purple blisters at the tip indicates Heart channel heat-toxin. Red tongue with purple macules in the center indicates impending rash eruption. Pale red tongue with red-red points in the center indicates impending jaundice. Red tongue cracking like the character 'Ren' indicates heat transmitting from Yangming to Shaoyin Heart. Pale red tongue cracking like the character 'Chuan' indicates accumulated heat in the Heart channel. Red tongue with prickles indicates heat accumulation in the Stomach. Red tongue with several black lines indicates Yin-Toxin binding in the Liver channel; since Liver governs sinews, tendril-like filaments appear on the tongue. Red tongue with double tongue indicates heat-toxin entering Pericardium; red tongue swollen filling the mouth indicates heat-toxin entering Shaoyang and Yangming; both require pricking to release bad blood. Red tongue bleeding like epistaxis indicates heat injuring Pericardium, requiring cooling blood and stopping bleeding. Red tongue stiff with loss of voice is a death sign; if phlegm, use guiding method; if interior excess, use purgation; survival is occasional. Red tongue rotting like insect erosion indicates Shaoyin plague-toxin, treat with purgation. Red tongue protruding and rolling indicates heat in Heart and Spleen. Trembling red tongue difficult to speak, or flaccid red tongue unable to speak, both indicate extreme Heart-Spleen deficiency, often salvageable with Ginseng. Red tongue dry and shriveled is death, untreatable. Pure purple tongue indicates alcohol toxin. Purple tongue with white center indicates alcohol toxin in Shaoyang. Purple tongue with red center indicates alcohol toxin in Yangming. Pale red tongue with several purple-black tendons in the center indicates Jueyin true cold syndrome. Based on this discussion, one can differentiate deficiency and excess, and distinguish life from death.