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From Zhang Shiwen's *Shanghan Xulun* (Treatise on the Sequel to Cold Damage), with slight annotations added based on my own views.

The term "tongue coating" (She Tai, tongue coating) originated from Changsha (referring to Zhang Zhongjing). Because pathogenic factors are knotted in the interior, as if something is gestating, it is called "tai" (fetus/germ). (Another term used is "tai", like moss growing on the ground.) When the pathogen of cold damage is at the exterior, no coating forms on the tongue. When pathogenic heat transmits to the interior, the coating gradually appears, turning from white to yellow, and from yellow to black; when black becomes severe, dryness and fissures appear. Generally, a slippery and white coating indicates exterior pathogenic factors; gray-black, moist, and without coating indicates yin-cold (this is actually a change in the color of the tongue body); gray-black, thin, and slippery indicates retention of cold food. In these cases, cold-cooling purgative formulas must not be used. However, in cases of summerheat combined with blood stasis, the center of the tongue is often black and moist, so one should not rigidly adhere to this rule. If the coating is yellow, black, or gray, dry, and fissured, it indicates extreme heat, and there is absolutely no possibility of deficiency-cold combined with blood stasis. (There are also cases where extreme heat followed by sudden ingestion of cold drinks causes blood stasis in the stomach region, resulting in a black coating.) Only after repeated sweating and purging, if the tongue is dry but has a very thin coating, without dryness, fissures, or prickles, does this indicate exhaustion of body fluids. It must not be mistaken for excess heat and treated with purgatives, otherwise death is inevitable. The thirty-six treatment methods in *Jingxian* (Golden Mirror) are revered by the world. Furthermore, the 138 diagrams in *Guan She Xinfa* (New Methods for Observing the Tongue) are detailed and well-organized in differentiation. Among them, red is discussed as epidemic heat, purple as alcohol toxin, moldy-soybean color as food retention, and blue as the pure color of the liver. These exceed previous records, but unfortunately, they lose sight of the root for the branches, becoming somewhat chaotic and lacking a guiding principle. Therefore, I have summarized their essentials and discuss them below:

If the coating is white, it indicates that the pathogen has injured the qi level. The lung governs qi, its color is white, and it governs the skin and hair. Therefore, any white coating carries characteristics of an exterior syndrome. Zhang Zhongjing believed that cold in the chest requires only harmonization and strictly prohibits purging; purging will inevitably lead to fullness and obstruction, with unpredictable complications. If it is warm disease or febrile disease, presenting immediately with high fever, stupor, restlessness, and thirst, with a red tongue body (this refers to the tongue body itself) covered by a white-slippery coating, Baizhu Tang (White Tiger Decoction) should be used to induce sweating. (Although Baizhu Tang can generate fluids and induce sweating, in the early stage, it is generally advisable to add a small amount of exterior-releasing herbs.) In the early stage of epidemic diseases, if the white coating on the tongue resembles accumulated powder, Dayuan Yin (Origin-Reaching Decoction) should be used to resolve it. If the pathogen of cold damage enters the stomach fu organ, the white coating turns yellow in the center; if the pathogen transmits to the Shaoyin channel, the white coating turns black. If the coating color is uniform, it is a syndrome of a single channel; if two colors appear simultaneously at the edges and center, it is a transmitting syndrome. If the tongue is divided straight into two paths from root to tip, it is a Hebing (Combined Disease) or Jia Yin (Clasping Yin) tongue. In Hebing, the white coating includes two paths of yellow; in Jia Yin, the white coating includes two paths of black-moist and gray coatings. If the tongue is divided horizontally into two or three sections of different colored coatings from root to tip, it is a Bingbing (Concurrent Disease) tongue. (The distinction between Hebing and Bingbing is ambiguous. Hebing refers to one pathogen injuring two channels simultaneously, or although transmitting from one channel to another, it remains cold pathogen, meaning two channels are combined under one pathogen. Bingbing refers to cold pathogen in one channel transforming into heat disease in another, or one channel having both cold and heat syndromes, meaning two pathogens concurrently affect the body. The straight division and horizontal segmentation of tongue coatings are unrelated to these concepts. Straight division into two or three paths divides by interior and exterior: the center is interior, the sides are exterior; the left corresponds to Liver/Gallbladder, the right to Spleen/Stomach. Horizontal division into two or three sections divides by Sanjiao: the tip is Upper Jiao, the middle is Middle Jiao, the root is Lower Jiao. Observe the color to determine cold, heat, deficiency, and excess.) Therefore, a white tip and yellow root, a white tip and black root, or a slippery half-tongue, although the symptoms differ, all belong to the Half-Exterior Half-Interior syndrome. More white coating and slippery, with less yellow/black coating, indicates more exterior syndrome, still suitable for harmonization. More yellow/black coating and less white coating, or presence of prickles, black spots, and dryness, indicates more interior syndrome, definitely requiring purging. Even if the center is yellow/black but the edges are slippery and white, this indicates the exterior syndrome is not fully resolved. (What is meant by "exterior syndrome not fully resolved" is that wind-cold has not completely transformed into heat. My view is that even if it transforms into heat, it still belongs to exterior pathogen, which is always different from true interior syndrome.) For cold damage, Dahuang Chaihu Tang (Major Rhabarber and Bupleurum Decoction) resolves both exterior and interior; for warm diseases and epidemics, Liangge San (Cooling the Chest Powder) or Baihu Tang combined with Chengqi Tang purges the interior. Additionally, in complicated cases of cold damage, if the coating is white but thick, even dry and fissured, this indicates the pathogen has consumed body fluids. Xiaochaihu Tang (Minor Bupleurum Decoction) with a little Mangxiao (Mirabilite) should be used for mild purgation. Pure white-slippery coating indicates deficiency-cold and fluid retention knotting above the diaphragm due to stomach deficiency. This often leads to changes after passing through the meridian cycle on the thirteenth or fourteenth day and should not be overlooked. Another type is a thick white coating that remains unchanged like cooked color throughout; this indicates retention of cold pathogenic substances in the interior that do not dissipate, leading to hidden pulse, indicating exhaustion of Heart and Spleen qi and injury to Lung qi. Purging must be strictly avoided. (For cold stagnation that does not transform, warm purging can be used.) Zhishi Lizhong Tang (Immature Bitter Orange and Spleen-Nourishing Decoction) is indicated; if heat is severe, combine with Xiaoxianxiong Tang (Minor Chest-Hamstring Decoction) to purge. As for those who can eat and have spontaneous diarrhea with white-slippery coating, this is Zangjie (Organ Knotting), which is difficult to treat. Huanglian Tang, Lianli Tang, or Beijij Wan may be selected, with occasional survival.

Yellow coating indicates Yangming Fu excess. Yellow, wet, and slippery indicates heat is not yet intense, and the knotting is not yet fixed; immediate purging should be avoided, as purging will lead to initial hardness followed by loose stools. In winter, this rule should be strictly observed, waiting until the knotting is fixed before purging; if unavoidable, use Dahuang Chaihu Tang for mild purgation. In summer, upon seeing a yellow coating, purging is appropriate. This is because summer involves latent yin inside the body, and there are many cases where purging is most urgent despite non-dry coating; one should not be dogmatic. If yellow and dry, heat is intense, and vigorous purging is indicated. Yellow with prickles and black spots indicates extreme heat intensity. Yellow with lobulated fissures indicates dryness of stomach fluids, making purging especially urgent. All yellow coatings belong to stomach heat; purging should be adjusted according to severity and urgency. There is a case where the root is yellow and hard, the tip is white, the middle is not very dry nor slippery, the tongue is short and cannot extend, accompanied by delirium and restlessness (different from the previous half-exterior half-interior syndrome). This is phlegm combined with retained food occupying the center. Dachengqi Tang (Major Order the Qi Decoction) with Shengjiang (Fresh Ginger) and Banxia (Pinellia) is indicated. There is a case where the tongue body is cyan-purple (this refers to the tongue body) but the coating is thick yellow, even fissured, feeling dry in the mouth but the tongue surface is not dry. This is yin syndrome combined with food retention. (Cyan-purple indicates blood stasis, not merely yin syndrome; it is a sign of damp-pathogen accumulation sinking deep into the blood level.) The pulse may be deep, fine, and hidden, or deficient, large, and choppy. Pressing the epigastrium or beside the navel reveals hardness and pain (phlegm knotting and blood stasis together often present with this pulse and symptom pattern), and frequent passage of gas. (Frequent passage of gas indicates either retained food/dry stool or qi collapse.) Urgently use Dachengqi Tang, decocting Sheng Fuzi (Raw Aconite) separately to assist Dahuang in purging. If the pulse is deficient and large, Huanglong Tang (Yellow Dragon Decoction) is indicated; for extreme heat and restlessness, add Shengdi (Rehmannia) and Maidong (Ophiopogon), especially suitable in summer. In winter, for yin syndrome combined with food retention with yellow non-dry coating, use Fuzi Lizhong Tang combined with Xiaochengqi Tang (Minor Order the Qi Decoction) to purge. Generally, if there is accumulated coating on the tongue, even if yin signs are seen, it is deficiency mixed with excess, requiring urgent purging, though the method differs from ordinary cases. Furthermore, if there is phlegm, fluid, water, or blood stasis in the center, the tongue coating is mostly not dry. Do not delay treatment because the coating is not dry. Whenever warm disease or febrile disease shows slight yellow-white coating, regardless of dryness or moisture, Liangge Shuangjie (Cooling the Chest Dual Resolution) is appropriate. For seasonal epidemics showing slight white coating, Baihu Tang and Dayuan Yin are appropriate. If yellow-black coating is seen, regardless of dryness or moisture, Dachengqi or Tiaowei Chengqi Tang should be used urgently to seize the pathogen.

Black coating is the color of Shaoyin Kidney. (This is extreme turbidity and stasis in the blood level. Dry and hard with faint purple indicates burning by heat causing blood deterioration. Soft and moist with faint pale color indicates water-fluid retention blocking qi circulation, causing blood stasis.) If after five or six days, heat transmits to Shaoyin, fire attacks the water position, the hyperactive fire, instead of being controlled by water deficiency, assumes water transformation (Five Elements theory is empty talk and a bad habit; this simply means heat pathogen has penetrated deeply, yin fluids are completely exhausted, blood is stasis and qi is turbid, presenting a withered and stagnant death color), like charcoal after fire. Initially due to failure to sweat for exterior syndrome (causing stasis), the pathogen enters Shaoyin; purging cures it. However, in cases of repeated purging where heat does not decrease and coating does not recede, this is due to retained food in the center. Huanglong Tang with Pao Jiang (Roasted Ginger) and Chuan Lian (Coptis) is indicated. If excessive sweating and purging were mistakenly used, leading to exhaustion of fluids (causing stasis) and dry black coating, this is a complicated disease. Treatment must be based on assessing deficiency and excess. Deficiency presents with thin, moist coating; Shengmai San combined with Fuzi Lizhong Tang is used. Excess presents with thick, dry coating; Shengmai San combined with Huanglian Jiedu Tang (Coptis Detoxification Decoction) is used. (Jiedu Tang is Sanhuang Tang.) One is yin deficiency with yang hyperactivity, the other is yang deficiency with yin hyperactivity; careful examination is required. If heat is extremely intense, prickles appear on the black coating, and it dries and separates into lobes. Use a blue cloth dipped in Bohe Tang (Mint Decoction) to wipe and moisten, then use ginger slices to scrape off prickles and lift the lobe edges to observe the base (the tongue body). If red, it is treatable; purge urgently. If entirely black, it is untreatable. Black coating rotting indicates exhaustion of Heart and Kidney; black tongue curling inward indicates exhaustion of Liver; both are untreatable. Black, gray, or yellow tongue with worm-like rotting, although due to damp-heat, also indicates Liver injury and is critical. Another type is a central moist black coating with dry surroundings, indicating moist tongue body but dry coating. Yellow or white on both sides indicates Lianggan (Dual Infection) tongue. (This might be pre-existing blood stasis, internal deficiency, external excess, not necessarily Lianggan. The text states Lianggan cases do not fit here.) Yellow edges suggest Tiaowei Chengqi Tang; white edges suggest Dahuang Chaihu Tang to purge. If the center has thin black-slippery coating and sides are white-slippery, this indicates both exterior and interior deficiency. Although there is retention in the stomach, Fuzi Tang should be used urgently to warm. Most black coatings are ominous (Heart qi blocked by blood stasis, pathogen溃 rapidly inside). Black, dry, or with prickles/fissures indicates excess heat, requiring urgent purging. Thin, moist black, or with white-slippery, indicates yin-cold, requiring urgent warming of meridians. A type with black, withered center, or slight prickles, black color but no accumulated coating in the center, withered and thin tongue shape, not very red (this is the tongue body), with symptoms of restlessness, thirst, deafness, continuous fever, constipation for 5-6 or 10+ days, abdomen not hard/full, painless on pressure, clear consciousness but unable to sleep day or night, muttering slightly when sleeping, smiling, or sighing, indicates exhaustion of fluids and dryness of blood. Jiugao Gancao Tang (Honey-Fried Licorice Decoction) or Shengli Liuwei Wan (Six-Ingredient Rehmannia Pill with Raw Rehmannia) combined with Shengmai San and Guizhi (Cinnamon Twig) to nourish the source of transformation is urgent. Mistaken use of Chengqi Tang leads to death; mistaken use of Sini Tang also leads to death.

Whenever the tongue coating is half-yellow half-black, half-yellow half-white, dry in center and slippery at edges, or dry at tip and moist at root, it indicates transmitting and concurrent pathogenic factors, and disharmony of cold and heat. Generally, black tip is slightly mild, black root is severe. Yellow-black requires Dachengqi Tang; if mixed with white, Liangge San is suitable. Purge according to urgency. If entirely black, it is a death-sign tongue, untreatable (complete blood stasis in Heart blood). In summer febrile diseases, with evil fire and seasonal fire burning internally and externally, black coating generates easily but can still be treated with purging. In winter cold damage, entirely black coating is difficult to save. (This refers to black and moist, where blood stasis is caused by cold; summer heat allows easier blood flow, winter cold makes blood stasis harder to move; if heat-stasis, both seasons are fatal.) However, if summerheat is mistaken for exterior sensation and treated with warming/recovering methods, the center often becomes black, edges very red and moist, pulse deficient and large. Baihu Tang should be used urgently to clear; add Renshen (Ginseng) and Zhuye (Bamboo Leaf) for deficiency. If further mistaken for yin-cold and given hot medicines, restlessness and death will result. Summer summerheat often presents with black tongue. Black with dry center indicates Baihu Tang is certain. Black with slippery/moist texture or white edges indicates cold-food retention. (Previously considered Lianggan or interior-exterior deficiency, here considered cold-food retention, which is more accurate. However, blood stasis was omitted. Blood stasis has cold and heat types, distinguished by coating moisture/dryness. Phlegm retention often shows gray coating, generally due to pathogen affecting blood level.) Ancient methods used Dashun San, but Lizhong Tang combined with Xiaoxianxiong Tang is more appropriate. If direct invasion of Shaoyin True Cold occurs, starting without fever, the tongue center is black (leading to coma/collapse), not changing from yellow/white. Coating is black but slippery, tongue is thin/small. This is True Organ Cold, with cold limbs, spontaneous diarrhea, vomiting, deep-slow pulse. Sini Tang and Fuzi-type formulas should be used urgently to warm; delay leads to untreatability.

Gray-black tongue indicates mutual disease of the three Foot-Yin channels. Mixing green-yellow into black creates gray. (Phlegm-water resides in vessels, causing slight blood stasis.) However, there is difference between transmitting and direct invasion. Transmitting heat pathogen starts as white, turns yellow, then gray-black, possibly with prickles, black spots, fissures, dryness, regardless of root or tip; purging and clearing heat is indicated. Light gray with deep black concentric rings in the center (due to long-standing cold-heat knotting, or pre-existing phlegm/fluid/blood stasis, plus new retention. If each transmission created one ring, it would be illogical; perhaps each transformation of pathogen into cold/heat/dry/damp adds a ring, or each injury from cold/heat/food/fluid adds a ring). This is warm disease heat toxin. Urgently use Liangge Shuangjie. Heat toxin transmitting once shows one ring; twice or thrice shows two or three rings. Three rings indicate untreatability. Direct invasion of Three Yin, starting without dry-heat, showing gray immediately, moist tongue without coating, not changing color, indicates internal cold-food retention, cold-phlegm/water-fluid, or blood stasis with mania. Treat according to syndrome. Also, cold with food retention, after repeated sweating/purging/digestion aids, with open bowels/bladder, but gray-black coating remains moist or not dry. Do not mistake moisture for cold and misuse Jiang/Fu Zi. Do not mistake lack of dryness for absence of need to avoid Xiaoshi/Huanglian. This is due to excessive sweating/purging damaging fluids, causing deficient fire to flare up. Pulse is deficient, minute, weak. Treatment urgently needs to nourish yin. Even without palpitations or intermittent pulse, Jiugao Gancao Tang is indicated. Contains Ganrun (sweet/moist) herbs like Shengdi, Ejiao, Huma Ren, Maidong to nourish yin and moisten dryness. Generally, if yang pathogen is hyperactive, use Xiaoshi/Huanglian to save yin; if yin-blood is exhausted, use Shengdi to nourish yin. Can this be ignored?

Red is the proper color of the Heart. Extreme red indicates warm-heat toxin accumulating in Heart/Stomach, or epidemic heat toxin internally盛. If moist, do not purge immediately; use Jiedu Tang or Baihu Tang. Red with white coating indicates contraction of out-of-season cold; Guizhi Baihu Tang is indicated. Red with two paths of gray coating indicates warm-heat with cold-food retention; Liangge San with one or two digestive herbs is indicated. Red with black coating indicates heat toxin entering Shaoyin; Dachengqi Tang combined with Baihu Tang is indicated. Extreme red with yellow-black prickles indicates heat toxin entering Fu organs; Tiaowei Chengqi Tang is indicated. Extreme red with purple-red macules and generalized maculation indicates Yang Du (Yang Toxin) entering Heart; Renshen Baihu Tang with Xijiao (Rhinoceros Horn) and Huanglian is indicated. Extreme red with fissures indicates dry-heat entering Liver; Dachengqi Tang with Chaihu and Baishao is indicated; add Qin and Lian if severe. Ulcerated/rotten indicates damp-heat entering Spleen; Xiaochengqi Tang with Qin, Lian, Banxia is indicated. White patches indicate fire blazing (superficial, not entering vessels, only appearing as white patches); Sanhuang Shigao Tang minus Mahuang is indicated. Purple ulcers indicate fire郁伏 (stagnant/deep); Jiedu Tang is indicated. Red stars (raised red beads) indicate Heart Pericardium fire flaming up; Liangge San is indicated. A type tender like new growth, looking moist but extremely dry/dehydrated internally, results from reckless sweating/purging exhausting fluids; mostly untreatable. Urgently use Shengmai San and Renshen Sanbai Tang. Tongue flaccid and immobile indicates Liver exhaustion; suddenly thin and elongated tongue indicates Heart exhaustion.

Purple indicates post-alcohol cold damage. Vulgar doctors and fools often, after catching cold, refuse herbal medicine, using ginger, scallion, and wine to sweat. If sweat is incomplete, wine toxin hides in the Heart Pericardium, often causing this syndrome. (Some transform into damp-warmth, some into wine toxin. The cause is cold binding the great exterior, wine power unable to exit outward, accumulating internally in Stomach and Pericardium.) Pure purple, or center slightly white and moist, suggests Gegen Tang (Kudzu Decoction) plus Shigao (Gypsum). Purple with red macules, or purple-dry-yellow, purple-shortened, all suggest Liangge San to purge. Entirely purple and dry, like cooked liver, is dead Liver color, with cold limbs and deep-slippery pulse (blood vessel stasis, yang depression failing to reach). This is extreme yang resembling yin. Urgently use Danggui Sini Tang (Angelica Decoction for Cold Extremities) plus Jiuhuang (Wine-processed Rhubarb) to purge. (Add Taoren/Peach Kernel.) However, mostly untreatable. (Danggui Sini Tang is still too tonifying; Huanglian is too draining. This syndrome requires dispersing, resolving stasis, unblocking vessels, opening blood, and reaching qi.) Generally, deep purple with red indicates yang-heat wine toxin; bitter-cold detoxification is suitable. (Focus on resolving stasis.) Pale purple with green-slippery indicates Kidney/Liver yin syndrome; Wuzhuyu Tang and Sini Tang should be used urgently to warm. (Should also resolve stasis.) However, there are cases with thin cyan-purple coating in center, or slight gray-black, neither dry nor wet, with urgent purging indications. This is heat pathogen injuring blood level; Xijiao Dihuang Tang plus Jiuhuang for mild purging is indicated. (Red and purple tongues refer to the tongue body; there is no such thing as red coating or purple coating. Those appearing purple are likely rotted tongue surface, or slight black coating reflecting against red, creating the illusion.)

Moldy-soybean color coating indicates cold damage with food retention. Food fills Taiyin, depressed and unable to vent, eventually becoming soybean color. Symptoms include abdominal fullness and intermittent pain; Guizhi Tang plus Zhishi, Houpo, Ju Pi, Banxia is used. Severe pain adds Dahuang. Due to undigested cold food, add Pao Jiang and Houpo; severely, Tiaowei Chengqi Tang plus Pao Jiang purges. If Stomach qi exhausts, pulse is knotted/intermittent, lips hang, teeth dry, with diarrhea, death ensues. (Note: This is also沉香色 (Sandalwood color). Always caused by blood stasis and turbid qi. Damp-heat with phlegm also commonly presents this, not just food retention.)

Blue coating indicates pure Liver color. (Vague.) Long-standing cold damage, repeatedly sweated and purged, neglected care, leads to extreme Stomach qi injury, Heart fire lacks qi, Spleen earth lacks foundation, thus Lung metal fails to generate; Liver wood lacks control, insults Spleen earth, so coating color is like indigo. Or accompanied by blue macules on body, indicating exhaustion of Heart, Spleen, Lung qi internally; death is certain. If light blue, not deep, or slight blue streaks, indicates Wood injured by Metal, organ qi not exhausted, pulse not deep-choppy but slightly wiry; treatable. (This statement is very reasonable but lacks elaboration. Deep-choppy means righteous qi does not arrive, pulse form is discontinuous/uneven. Slightly wiry means qi arrives but blood blocks it, hence tense pulse form.) Xiaochaihu Tang plus Rougui (Cinnamon Bark) and Pao Jiang is indicated. (Note: Commonly seen in epilepsy/collapse and chronic Stomach pain, whole tongue is blue. This is also blood stasis in Stomach, Liver qi not soothing. Thus cyan, black, blue, crimson are all called turbid, involving blood level. Must differentiate cold/heat, dry/damp, phlegm/blood, retained food/dry stool, masses, treating primarily by loosening blood level.) Although tongue syndromes are numerous, they do not exceed eight coating colors. Summarizing the essentials assists in differentiation. (Zhang Shiwen, *Shanghan Xulun*.)