Classica Sinica Medica

不可挂以髮 简体 不可挂以发

bù kě guà yǐ fà

Not a hair's breadth · bu ke gua yi fa

Also rendered: not a hair's breadth (Idiomatic English for the same margin) · nothing as thin as a hair may be hung on it (Word-for-word, and rather beautiful)

Core vocabulary

Reviewed · 2026-09

At a glance

Pinyin
bù kě guà yǐ fà
Simplified
不可挂以发
Literal
挂 is 說文's 畫, to mark or divide — the hand making a boundary — and by extension to hang something on. 髮 is the hair of the head, 說文's 頭上毛. So the phrase pictures something so fine that a single hair cannot be hung on it: a margin too small to measure. Chapter 27 uses it twice in the space of four lines, and it is the interval the whole timing argument comes down to. Wang Bing's gloss is that you must not let a thing as tiny as a hair make you miss the moment — the window in which the evil can be taken is exactly that narrow.
Shuowen
《說文解字》：挂，畫也。从手圭聲；髮，頭上毛也。从髟犮聲。不可挂以髮，言其間細微至不容一髮，喻時機之不可差失。王冰注：不可挂以髮者，謂不可取髮之微小而誤其時也。本篇之目：不可挂以髮者，待邪之至時，而發鍼冩矣。

In brief

Too fine for a hair to hang on — the margin by which the moment can be missed. Chapter 27 states it twice within four lines, once as a definition (“not to be hung by a hair” means waiting for the evil’s arrival and then releasing the needle) and once as the mark that separates the physician who knows the trigger from the one who does not. It is the concrete form of the chapter’s two prohibitions: the evil’s coming cannot be met, its going cannot be pursued. Between those two, everything depends on a hair.

Senses in the classical corpus

1
The definition 不可挂以髮者，待邪之至時
間不容髮，待其至而發針
不可挂以髮者，待邪之至時，而發鍼冩矣 — Suwen 27
2
What being off costs 若先若後者，血氣已盡
或先或後則血氣已盡，病不可下
若先若後者，血氣已盡，其病不可下 — Suwen 27
3
The one who knows 知機道者，不可挂以髮
知機者不差一髮
故曰知機道者，不可挂以髮 — Suwen 27
4
The one who does not 不知機者，扣之不發
不知機則扣而不發
不知機者，扣之不發 — Suwen 27
5
The earlier form of the same rule 其來不可逢
來不可逢、往不可追，即其間不容髮之故
經氣太虚，故曰其來不可逢 — Suwen 27
6
The image of knowing 如發機
知取如發機，不知取如扣椎
故曰知其可取如發機 — Suwen 27

Occurrences in the classics

Passages where the term does real work, with the original, pinyin and English on the same line.

Display
1 passages
素問·離合真邪論 Suwen 27 · 離合真邪論 Read in context →
不可挂以髮者，待邪之至時，而發鍼冩矣。若先若後者，血氣已盡，其病不可下。 不可挂以发者，待邪之至时，而发针写矣。若先若后者，血气已尽，其病不可下。
Bù kě guà yǐ fà zhě, dài xié zhī zhì shí, ér fā zhēn xiè yǐ.Ruò xiān ruò hòu zhě, xuè qì yǐ jìn, qí bìng bù kě xià.
“Not to be hung by a hair” means this: wait for the moment the evil arrives, and then release the needle and drain. If one is early or late, the blood and qi are already spent, and the disease cannot be brought down.
「不可挂以髮」是全篇論時機的結穴，而上文「其來不可逢」「其往不可追」兩句是它的根據——來不可逢，往不可追，那麼可取的就只有「至時」那一刻，而那一刻的寬度，據本篇說，連一根頭髮都挂不住。**「挂」讀 guà，本篇二見**，底本作挂不作「掛」；**「髮」讀 fà，本篇二見**，底本作髮不作「发」；**「發」讀 fā，本篇三見**（此處、「如發機」、「扣之不發」），與「髮」二字分用不混，底本亦然。「若先若後」（「先」讀 xiān，「後」讀 hòu，底本作後不作后）是失時之狀：不是差得很遠，只是先了一點或後了一點，而已經「血氣已盡，其病不可下」。下文「如發機」「如扣椎」之比（「機」讀 jī，「扣」讀 kòu，「椎」此讀 chuí）即由此而生：弩機一觸即發，木椎扣之則不發。故本篇之「機」不是玄談之機，而是扳機之機——一個有形的、按下去就動的東西。

Compounds built on this term

不可挂以髮
bù kě guà yǐ fà
not a hair's breadth
本篇二見，一言其義，一言其人
發機
fā jī
releasing the trigger
機讀 jī，弩機之機
扣椎
kòu chuí
tapping a mallet
扣讀 kòu，椎此讀 chuí
若先若後
ruò xiān ruò hòu
whether early or late
先讀 xiān，後讀 hòu

How it has been translated

Every rendering below is a decision about which reader is being addressed. There is no neutral option.

Wiseman & Fengone cannot be off by as much as a hairThey render the sense rather than the image, which is right for a clinical instruction and loses the hanging hair.
Unschuldone must not let a hair's width come betweenUnschuld keeps the hair as a measured interval rather than as something hung, which is closer to Wang Bing's gloss.
This sitenot a hair's breadthWe use the English idiom because it carries exactly the meaning the chapter needs — an interval too small to act within — and because the phrase has to work twice in four lines, once as a definition and once as the mark of a physician. The literal 'hang a hair on it' would need a gloss both times.

Entry status

Review pending — who reviews this?

2026-09

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Reading path

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