Every other subject on this site is something the palm leaf keeps. This one is something it does not. The blue shirt is the single most legible piece of Lanna material culture in the world — it is what "northern Thai" looks like on a poster — and across 6,986 catalogued manuscripts there is not one recipe for making it. That absence is the page.
ห้อม hom /hôm/ is the plant: Strobilanthes cusia (Nees) Kuntze, of the Acanthaceae, a shade-loving shrub of the northern hills, published for a century under the synonym Baphicacanthus cusia and still sold under it. In the north it is written ฮ่อม, and the two spellings are the same sound: ห is a high-class consonant, so ห้อม with mai tho falls; ฮ is low-class, so ฮ่อม with mai ek falls to the same place. Northern orthography reaches for ฮ as a matter of habit — it is the letter kam mueang uses where central Thai has ร (รัก → ฮัก, เรือน → เฮือน) — and the plant got carried along. It is not the lowland dye plant. คราม khram /khraam/ is Indigofera tinctoria, a legume of the plains and the northeast, and the two are in different families, on different ground, worked by different people. Thai prose uses คราม as the generic for the colour and the process; this page follows the north and keeps ห้อม for the plant.
Terms are given four ways where it helps: script — RTGS — /precise, with tone and vowel length/ — then the root or the morpheme split. Tone: à low, á high, â falling, ǎ rising, unmarked mid; doubled vowels are long.
The garment is หม้อห้อม — หม้อ mo /mɔ̂ɔ/ "pot" + ห้อม — the pot of indigo: a piece of clothing named after the vessel it was made in. Which is odd enough that a rival etymology exists, and the state has had to rule on it.
Signs in Phrae say ม่อฮ่อม. Signs in Bangkok say หม้อห้อม. Both are pronounced identically in Bangkok Thai, and the fight over which is right is about a hundred and fifty kilometres of vowel.
Tradition holds — the ม่อ camp derives the first syllable not from หม้อ "pot" but from มอ mo /mɔɔ/, an old word for a dull dark colour — so ม่อฮ่อม would mean "indigo-dark", a colour word, and the pot would be a folk etymology laid over it afterwards.
The Office of the Royal Society has ruled against that. Its position is narrow and worth stating exactly: ม่อฮ่อม should not be written, not because the colour etymology is wrong, but because it is not a correct transposition of the Payap (northern) sounds into Bangkok spelling. Bangkok Thai collapses ม่อฮ่อม and หม้อฮ่อม into one pronunciation — both come out /mɔ̂ hɔ̂m/ — while northern speech keeps them apart on the length of that first vowel: หม้อ the pot is long, ม่อ the colour-word is short. The dictionary entry that stands defines หม้อห้อม as a round-necked, short-sleeved shirt, opening down the front, usually dyed deep blue or black, and notes ม่อห้อม and ม่อฮ่อม as variants in use. The Royal Institute had already taken the question up in a 1975 bulletin. Inference — a ruling that has to be re-issued for fifty years is a ruling the signwriters are ignoring, and they are: the shops at Thung Hong overwhelmingly paint ม่อฮ่อม.
Thirty-seven witnesses name cloth. Not one of them names a colour.
The distribution is the finding. Twenty-five are canonical merit texts — the anisong, the reward-of-giving genre — and what they give is cloth: the kathin robe, the rains-bathing cloth, the rag-robe, the banner. Five are filed under astrology, two are chronicle, two ritual, three miscellaneous. And four bundles — three of the astrological ones plus one the catalogue files as other — are about when you may cut cloth. The scribes wrote about cloth constantly. They wrote about giving it, about the day to cut it, about the merit that comes back. They never once wrote down how to make it blue.
Inference — this is not an accident of survival. Palm leaf in Lanna is a monastic technology: it is copied in a temple, stored in a temple, and its subjects are the subjects a temple has a use for. Dyeing was women's work, done in a yard, taught by hand, and it had no reason to enter that stream. The same silence covers weaving, cooking and midwifery. The catalogue is not a record of what Lanna knew. It is a record of what Lanna's monks copied.
Cut a stem of ห้อม and it is green, and it stays green. There is no blue pigment anywhere in it. What the leaf carries is indican — indoxyl bound to a sugar, 0.4–1.3% of the leaf by weight — which is colourless and water-soluble and does nothing on its own.
The blue has to be manufactured in three steps, and traditional practice performs all three without naming any of them:
1. Hydrolysis. Steep the cut stems under water for a night. A glucosidase in the plant splits indican into glucose and indoxyl. The liquor goes yellow-green and stinks. At Thung Hong this is a night and a turn — flip the bundle in the morning so the top leaves go under, and draw off in the afternoon: eighteen to twenty hours. 2. Oxidation. Strain the leaves out and beat lime into the liquor. Indoxyl pairs with itself in air and becomes indigotin, C₁₆H₁₀N₂O₂ — which is blue, and which is insoluble in water. It falls out as a paste, เนื้อคราม nuea khram, the colour of a bruise. Ban Khan Phalan burns cockle shell for its lime and reckons about ten grams to the litre. 3. Reduction. And now you have a problem, because a pigment that will not dissolve cannot dye anything. The paste must be put back to a soluble form — leuco-indigo, indigo-white, which is a yellow-green dianion — and held there, in an alkaline liquor, by bacteria that eat sugar and breathe the pigment. That liquor is the pot.
So the dyer dips cotton into a yellow-green bath and lifts out yellow-green cloth, and it turns blue in her hands as the air gets at it. Every account of indigo dyeing that has ever been written stops at that moment, because it is the only step in the whole of textile practice that looks like magic and is not.
ย้อมเย็น yom yen — "cold dyeing" — is the trade's own term, and it exists because indigo is the exception. Every other natural dye in the northern repertoire is extracted by boiling and taken up hot. Indigo cannot be: heat does not dissolve it, and the vat's bacteria die. It is the one dye you must keep at blood heat and no hotter, which is why the pot stands raised off the ground where morning and evening sun can reach it.
This is the part that belongs to this site rather than to a chemistry book.
A reduction vat is a microbial culture. It has a population, an appetite, a metabolism and a death, and the vocabulary that grew up around it in Thai is not metaphorical language for a chemical process — it is the language you would use for an animal, applied because nothing else fits.
In Isan the vessel is หม้อนิล mo nin /mɔ̂ɔ nin/, and building one is ก่อหม้อนิล ko mo nin — to found a pot, the verb used for laying a fire or raising a wall. Tradition holds — the pot is inhabited: a spirit lives in it, and the practice around it is care, not procedure. The recorded vocabulary of its states, gathered from dyers in Ubon Ratchathani in 2006, reads:
And a pot that has run can come back: a lak-ni pot that recovers is said to have become pen again, which is the same word Thai uses for a person who survives.
The feeding is literal. Wood-ash lye — น้ำด่าง nam dang, filtered a dipper at a time through ash tied in cloth, and tasted, because the old rule is that lye which is not salty is thrown away and never used. Half a ripe กล้วยน้ำว้า banana. Small pinched balls of tamarind. A ladle of paste. A dipper of liquor begged from a pot that is already alive, as a starter — the word used is เชื้อ chuea, the same word as a yeast culture, or an infection, or a lineage. Three or four times a day the dyer performs โจกคราม chok khram: lift the liquor and pour it back through the air. Then, once a day, about a pickled-vegetable tin of lye, so the pot does not die.
The instrument reading is exact. A new vat runs at pH 12.5–13 and drifts down to 10–10.5 over roughly twenty days, and the whole apparatus of banana and tamarind and ash and daily pouring is an unwritten titration performed by tongue and eye. One pot yields one dip a day. A dark shade wants many, so a serious dyer keeps several pots, and four or five days of dip-and-cure. After each dip the yarn is wrung and left overnight in a covered vessel to cure before it is rinsed: dipped in the morning, cured till evening; dipped in the evening, cured till morning.
นิล nin /nin/ is Sanskrit and Pali nīla — dark blue, blue-black; the same syllable that gives Thai the black gem. It is also the exact word in which the Vinaya forbids a colour to a monk.
The Mahāvagga allows six classes of dye and no others — root, stem, bark, leaf, flower, fruit (Mv.VIII.10.1) — and then rules that robes entirely nīla may not be worn, along with those entirely yellow, blood-red, crimson, black, orange or beige (Mv.VIII.29). The commentarial discussion, when it comes to leaf-dyes, singles indigo out. What is left after the exclusions is the dull red-brown of bark, and that is the robe.
Inference — two things follow, and the second is the one worth having. The first is trivial: indigo is a leaf-dye, formally allowed by the six-fold rule and knocked out by the colour rule. The second is that a lay village and a monastery standing fifty metres apart, dyeing cloth in the same water with the same plants, arrived at opposite colours by the same text — and the peasant's pot is named, in the dialect that has least Pali in it, with the Pali word for the colour a monk may not wear. The blue shirt is not indifferent to the robe. It is the robe's complement, and it is spelled that way.
The Vinaya explains why a monk cannot wear it. It does not explain why a farmer wants to — and the answer to that is not comfort, and not fashion. It is planetary.
นิล is not only a colour word. In the นพรัตน์ nopparat — the nine gems, nava-ratna, which are the nine planets worn as stones — the dark blue stone is นิลกาฬ ninlakan (← nīla + kāḷa, blue + black), the blue sapphire, and it is พระเสาร์'s. Saturn. The nine-gem ring, the นพเก้า, is the single most-worn auspicious object in Thailand and every stone in it is a planet; the indigo one is the planet the tradition treats most carefully.
Tradition holds — Saturn's own deva is described as dark-bodied and adorned with ไพลิน and นิลรัตน์ — sapphire and nil-stones. He wears the colour. And the prescription for his day is stated in the same Indic vocabulary the Vinaya uses: Saturday's ornament is กัณหาภรณ์ kanhaphon ← Pali kaṇha "black" + ābharaṇa "ornament".
Which closes a circle worth stopping on. kaṇha is on the Mahāvagga's list of robe colours a monk may not wear, one line from nīla. The astrologer prescribes for Saturday exactly what the Vinaya forbids to the robe — and the whole week agrees with the astrologer: the everyday scheme gives Saturday ม่วง, purple, and the campaign list in Sawatdirak, which the manual in this collection reprints, gives Saturday ดำ, black (Compendium of Katha from Old Manuals, pp. 15–16; magic & ritual). Two lists for two occasions, both putting the dark end of the spectrum on Saturn's day.
Inference — so the blue is not neutral and it is not merely practical. It is a planetary colour in a system where colours belong to planets, worn by people who know that. And when the Isan dyer names her vat หม้อนิล she is using the gem's name — the pot is called after Saturn's stone. That may be nothing more than the ordinary Thai word for blue-black doing ordinary work. It may also be why a pot has a spirit in it at all, when a rice pot does not. The corpus cannot settle that; it is recorded here as the question.
One witness ties the two ends together on a single folio. The Phayao bundle Sado kho, Tam khwan cao nai … Wen tan pha lwat at Wat Si Khom Kham holds สะเดาะเคราะห์ — the rite that undoes a bad turn in the chart — the soul-calling for a lord, the New Year offering of scented waters, and เวนทานผ้าลวาด, the giving of the pha lwat cloth. Fate-remedy and cloth-gift, copied by one hand into one bundle. It is also one of the thirty-seven.
Ask what indigo wards off and the tradition answers in three different registers, and they do not agree with each other. That disagreement is the interesting part, so they are kept apart here.
The empirical one is real and is not folklore. Indigo's own molecules — tryptanthrin and isatin, which ride into the cloth with the pigment — are antibacterial, antifungal and anti-inflammatory; the dye smells wrong to insects; and indigo cloth is measurably good against ultraviolet. Every tradition that works the pot has found this out independently. Tradition holds — the Lanna and Isan version is a farmer's: an indigo shirt is why a man working sun-up to sun-down in open paddy does not burn, and the cool paste itself, เนื้อคราม, goes onto a centipede or scorpion sting to draw the poison and take the pain down. Japan reached the same place from the other side — samurai wore indigo under armour against wound infection, and the fire brigades wore it for the way it resists a flame. A cloth that keeps out sun, insects, rot and infection is a protective garment before anyone says a word over it.
The hill-country one is explicit. Tradition holds — among the Hmong, and in varying form among the Akha, Mien and Lisu who work the same plant on the same slopes, indigo is a ward. The batik and the embroidery laid over it are built as a field that turns evil aside, and the most heavily worked pieces in the whole repertoire are the ones made for infants — the baby carrier and the baby cap, the two things a spirit would come for. Some caps carry eyes, to be looked back at. Inference — the logic is the oldest one there is for a dark colour: what cannot be picked out cannot be taken, and a child dressed the colour of dusk is a child a spirit walks past. This is not a Lanna tradition and this page does not claim it as one — but the ห้อม on those slopes and the ห้อม in the Phrae pot is the same plant, cut on the same hills.
And the Thai amulet tradition says no. This is the corrective, and it matters more than the two claims above. If indigo were the protective colour of central Thai magic, the protective cloth would be indigo. It is not. ผ้าประเจียด pha prachiat — the inscribed cloth tied round the head or the upper arm, the ancestor of the muay thai armband — is cut from ผ้าสาลู, plain white, and worked in white, red and yellow. The yantra cloth is white because a yantra has to be read; ink on indigo is ink on nothing (yantra). So the drawn magic and the dyed magic are two different technologies with two different colour rules, and the corpus's 37 cloth witnesses sit almost entirely on the drawn side — which is one more way of saying the same thing this page has said throughout. The leaf kept the side of cloth that could be written on.
These four are the ones nobody expects, and they are the argument for reading a catalogue rather than searching it. Three sit under astrology and one under other, which is itself worth noticing: the classifier could not decide whether a cloth-cutting day was a fortune-telling text or a miscellany, and that hesitation is the right one.
ตัดเสื้อผ้า tat suea pha — "cutting clothes" — is an auspicious-day question, and four Lanna bundles hold the rule: Tat sue pha, Liang phi, Pi ao phua ao mia at Wat Wiang in Lampang, which files the cloth-cutting day beside the day to feed the spirits and the year to take a husband or a wife; an untitled Lampang bundle from Wat Ban Luk that runs wan tat suea pha, wan nung pha, wan mai, wan lokawinat — the day to cut cloth, the day to put it on, new year's day, and the day the world is destroyed, in that order; and a Phayao compendium at Wat Si Khom Kham that sets it after the day to first eat the rice and the naming of oxen and buffalo.
The fourth is the one to read twice. Tat suea pha, Ngua, Hung, Maeo, Suea, Latcasi, Nak, Cang, Nu, Wat Wiang, Lampang: cutting clothes; ox; rainbow; cat; tiger; lion; otter; elephant; rat. A garment sits in a list of omens — one row among the animals whose appearance means something. Inference — this is the register the leaf puts cloth in. Not craft, not commerce: reading. Cloth is a thing that can be timed wrongly, like a marriage or a housepost, and the manuscript that knows this is the same manuscript that knows what a rainbow means. The elephant is in that list too, one item along (the elephant) — the same folio, the same eye.
The merit texts hold the other half. Ten witnesses across five provinces carry ผ้าบังสุกุล pha bangsukun ← Pali paṃsukūla, paṃsu "dust" + kūla "heap" — the rag-robe, cloth taken from a dust-heap or a corpse and made into a robe, the most ascetic garment the tradition has and the one every Lanna scribe seems to have copied the merit of: Wat Phra Sing in Chiang Mai, Wat Lao Noi in Lampang, Wat Si Khom Kham in Phayao, Wat Phothanaram in Chiang Rai, and a Lamphun copy. Beside them the kathin robe, the rains-bathing cloth, the banner-cloth on its pole, and the mouth-and-head wiping cloths offered with the monastery buildings.
And the two witnesses that contain the word ย้อม yom /yɔ́ɔm/, "to dye", are neither of them about dyeing cloth: Katha long nyan pha sahang lae nyom kham hue ngam at the Nan Provincial Museum is a spell for laying yantra on cloth, bundled with one for ย้อมฅำหื้องาม — making gold beautiful, the Lanna ฅ still standing in the title; and a Wat Chiang Man bundle whose second title the catalogue reads as Sut nyom simma, a formula touching the ordination boundary. The corpus's only two dye-words are about gold and about ground.
Inference — the counter on this page will move if the crawl ever reaches a household manual rather than a monastery shelf, and the aliases are already set to catch it. Until then, 37 is not a small number for cloth. It is a complete number for cloth-as-the-temple-saw-it, and zero is the complete number for colour.
Ask in Chiang Mai and you get told, correctly, that people wear mo hom on Fridays. The reason is not old, and it is not northern, and it is written down.
Cabinet resolution of 9 June 2020 (BE 2563) approved measures to promote Thai cloth, of which the first asks officials and the public to dress in Thai or local cloth at least two days a week — on Tuesday and Friday. The Ministry of the Interior pressed provincial governors to drive it; ministries wrote their own versions; the guidance to a department is to declare a Thai-dress day on a Friday or a Tuesday, or more often. What each province wears is left to the province, so the policy is one and the garments are many: the north puts on mo hom, and Sakon Nakhon puts on ผ้าย้อมคราม, its own indigo, from the other plant — the northeast keeps the same Friday with Indigofera where the north keeps it with Strobilanthes. Mukdahan wears Mukdahan cloth. The rule is national; the blue is local twice over.
That is the whole documented answer, and the habit is older than the resolution in practice — provincial and departmental Friday dress days ran for years before 2020, which is what the resolution was tidying up. This page has found no earlier instrument that fixes the day, and says so rather than inventing one.
Inference — but the day is not arbitrary either, and here the two halves of this page meet. In the สีประจำวัน system — the auspicious colour of each weekday, which Thai tradition derives from the body-colours of the เทวดานพเคราะห์, the nine planetary devas — Friday is สีฟ้า, blue, from พระศุกร์, Venus. So a northerner in an indigo shirt on a Friday is doing two unrelated things at once: complying with a 2020 administrative measure, and wearing the day's own colour. Nobody had to arrange that. It is the kind of coincidence a living tradition absorbs and then explains, and within a generation the second reason will be the one people give.
Two cautions on that. First, the state's reasoning is documented and it is about craft income and community markets, not about Venus; the alignment is real but it is not a cause.
Second — and this one is a correction to an earlier version of this page, which got it wrong and was caught by a reader who wears these colours. Do not reach for the manual's list here. The wardrobe rule in Compendium of Katha from Old Manuals (pp. 15–16) is Sawatdirak, and Sawatdirak opens the passage อนึ่งภูษาผ้าทรงณรงค์รบ — ให้มีครบเครื่องเสร็จทั้งเจ็ดสี: "as for the cloth worn going into battle, have all seven colours complete." It is campaign dress, worn on the day you fight, applying whatever day you were born on. Its seven are red, white, purple-with-indigo, orange-red, yellow-green, cloud-grey, black.
The everyday scheme is the other list, and it is the one that governs what anyone puts on for a Friday: Sunday red, Monday yellow, Tuesday pink, Wednesday green, Thursday orange, Friday blue, Saturday purple. That is what people wear and what they will tell you is traditional, and they are right.
Inference — setting those two lists side by side and calling the difference historical drift is comparing a helmet with a shirt. Both trace themselves to the same มหาทักษา root — the body-colours of the planetary devas — and they land differently because they are for different occasions. Which came first is not something this page can settle, and it no longer pretends to. What survives, and is worth keeping, is narrower and firmer: on both lists the dark end of the week belongs to Saturn — black in the campaign list, purple in the everyday one — and indigo appears in the campaign list on Tuesday, as war-cloth. A practitioner following Sawatdirak wore indigo to fight in. He did not wear it on Tuesdays for luck.
บ้านทุ่งโฮ้ง Ban Thung Hong, Tambon Thung Hong, Amphoe Mueang, Phrae — 18.174 N, 100.166 E, five muban, about 4,840 people in 2,037 households — is where the world buys the shirt. It is a Tai Phuan (ไทพวน) village. Its people came from Muang Phuan in Xieng Khouang, in what is now Laos, moved south by the Siamese redistribution of Phuan populations among the armies that had fought in the Lao campaigns: seventeen families into Nan in 1834 (CS 1196 / BE 2377), then relatives after them, over decades.
Tradition holds — the name has nothing to do with dye. It is ทั่ง thang /thâŋ/, an anvil — a place of hammering — plus โห้ง hong /hôoŋ/, Phuan for a deep hollow: Ban Thang Hong, the smithy in the basin, worn down to Thung Hong. The village that clothes the north in blue is named after iron, and its oldest recorded taboo is a smith's: the forge spirit ผีเตาเหล็ก had to be propitiated annually or it turned into a ผีกะ, so households now raise a temporary forge for a job and pull it down after, to give the spirit nothing to settle in.
They also keep กำฟ้า kam fa — the sky rite, January or February, three to five days in which no work is done at all, listening for the direction of the first thunder and reading next season's rain out of it. Inference — a Phuan village keeping a Phuan sky-rite two centuries after the march is the same mechanism that kept the pot: displaced people hold their technique harder than settled ones, because it is the part of home that travels.
Since 6 August 2005 the natural-dye work at Thung Hong has been organised as วิสาหกิจชุมชนหม้อห้อมทุ่งเจริญย้อมสีธรรมชาติ, a registered community enterprise; the cloth has been an OTOP handicraft-tourism village; and on 30 March 2020 ผ้าหม้อห้อมแพร่ was registered as a Thai geographical indication, which fixes in law what had been a claim — that the name belongs to Phrae, and to indigo grown there.
The shirt is old. The uniform is not, and it has a date and an address.
Tradition holds — in 1953 (BE 2496) ไกรศรี นิมมานเหมินท์ Kraisri Nimmanhaeminda, then the northern regional director of a Bangkok bank and the man who turned a household meal into a public form, gave a ขันโตก khantoke dinner in Chiang Mai in honour of Sanya Dharmasakti and the American consul — and set a dress code: a round-necked cotton shirt, indigo-dyed, with a ผ้าขาวม้า phakhaoma at the waist. It was copied, and copied, and it is now what a northern man wears to a formal northern occasion.
Inference — three consequences, all visible today. The shirt entered polite society as costume rather than as clothing, which is why it is worn to festivals and Fridays and not to work. It was formalised in Chiang Mai and manufactured in Phrae, which is why the province that made it famous is not the province that sells the most of it. And it was chosen by a man who was cataloguing Lanna at the same time — Kraisri is a founding figure of northern Thai studies — so the garment arrived already framed as heritage, by someone qualified to frame it. The road in Chiang Mai that carries his family's name is the one the boutiques are on.
That framing narrowed the thing. The reference works are plainer: an indigo round-necked shirt is documented for Tai Lue of Sipsongpanna, for Lao in Laos and the Thai northeast, and for Tai Yuan in the north. It was, before it was Phrae's, simply what Tai people wore to work in.
Adolf von Baeyer cracked indigo's structure in 1878 and spent nineteen years and more than BASF's stated capital getting to a process that paid. Synthetic indigo went on sale in 1897, into such disbelief that BASF hired an outside firm to certify it. Within a decade about ninety-five per cent of the natural indigo trade was gone; a world crop of roughly nineteen thousand tonnes was under a thousand by 1914. Baeyer took the Nobel in 1905.
What survived that was not the industry. It was the pot — in exactly the places where nobody was selling into the world market, which is to say northern villages dyeing for themselves. Inference — the reason Thung Hong still has the practice is that in 1897 it was not worth destroying.
And the test that separates the two is beautifully backwards. Real indigo-dyed cloth crocks. It runs in the first washes; it fades; it wants re-dyeing; the tradition's own advice is to wash it out several times and expect the water to run blue, and to accept that the garment will get better looking as it goes. The synthetic dye does not do this — its colour holds. So the flaw is the proof, and a mo hom shirt that never bleeds is a printed one. This is the single most useful fact on this page for anyone standing in a shop, and it is worked through, with prices and the walk, on the living-city side at motdang.net/hom.html.
ห้อม is not only a dye, and its medical career is older and better documented than its textile one.
The root and rhizome of Strobilanthes cusia are 南板藍根 nán bǎn lán gēn, "southern ban-lan-gen", a Chinese materia medica with about a thousand years of use — for fevers, swelling, inflammation, and in the modern pharmacopoeia for viral complaints. The leaf and stem are Malanye, southern daqingye, an antipyretic. And the processed pigment itself is 青黛 qīngdài, indigo naturalis, applied to skin disease. In northern Thailand the Mien (Yao) use the leaves against diarrhoea.
The village pharmacology is the same knowledge from the other end. Dyers say the indigo in the cloth is why a farmer working sun-up to sun-down does not burn, and that the cool paste is what you put on a centipede or scorpion sting to draw the poison and take the pain down. Inference — a plant that is simultaneously a textile, a fever medicine and a poultice is exactly the profile the medicine manuscripts record for a dozen other species, and its absence from them is a gap in the catalogue rather than in the tradition.
The living-city directory for Chiang Mai and Chiang Rai held 158 craft and tailoring shops on 31 August 2026 and not one of them had ห้อม in its name — the trade is real and the signage is not, because the shirt is sold as a shirt. What the map does keep is the plant. Three watercourses are named for it — ห้วยห้อม Huai Hom in Mae Ngon, Fang (Chiang Mai) and again in Pa Tan, Khun Tan (Chiang Rai); ห้วยฮ่อม in Ban Chan, Galyani Vadhana — and a hamlet, บ้านเด่นฮ่อม in Bo Kaeo, Samoeng — four in all, and the sibling counts them live, so that number is the one to trust. Inference — Northern place-names take the form ห้วย + plant, and a creek called Indigo Creek in three separate valleys is where the plant grew thickly enough to name the water. Guard the reading, though: ฮ่อม in kam mueang is also an ordinary word for a lane.
The drawn version of everything on this page — the migration route from Xieng Khouang, the chemistry as a cycle, the pot's four states as a diagnostic dial, and the timeline from 1834 to the GI — is at motdang.net/hom.html. This page keeps the tradition; that one keeps the walk.