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A hair's breadth, or the width of human hair, is used as an informal unit of a very short length. It connotes "a very small margin" or the narrowest degree in many contexts.
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Until the middle of the 20th century, the highest resolution of measurement was considered to be the same order of magnitude, around 10−5 metres, as the diameter of a human hair. A "hair's breadth" was, and still is, informally, a very small measurement.
Definitions
This measurement is not a precise one. Human hair varies in diameter, ranging anywhere from 30 μm to 100 μm. One nominal value often chosen is 75 μm, but this – like other measures based upon such highly variant natural objects, including the barleycorn – is subject to a fair degree of imprecision.
Such measures can be found in many cultures. The English "hair's breadth" has a direct analogue in the formal Burmese system of Long Measure. A "tshan khyee", the smallest unit in the system, is literally a "hair's breadth". 10 "tshan khyee" form a "hnan" (a Sesamum seed), 60 (6 hnan) form a mooyau (a species of grain), and 240 (4 mooyau) form an "atheet" (literally, a "finger's breadth").
Some formal definitions even existed in English. In several systems of English Long Measure, a "hair's breadth" has a formal definition. Samuel Maunder's Treasury of Knowledge and Library of Reference, published in 1855, states that a "hair's breadth" is one 48th of an inch (and thus one 16th of a barleycorn). John Lindley's An introduction to botany, published in 1839, and William Withering' An Arrangement of British Plants, published in 1818, states that a "hair's breadth" is one 12th of a line, which is one 144th of an inch (a line itself being one 12th of an inch).
Red cunt hair
A subset of "hairs breadth" is the red cunt hair or RCH, red pussy hair, and red one which is a figure of speech used to represent very small widths. It is a slang, tongue-in-cheek term that purports to describe a unit of measure, but is actually a subjective notion, and can change depending on the situation in which it is used.
The term is based on the concept of a pubic hair being small in diameter, and can be used to describe a minor adjustment necessary, and is akin to terms such as 'a tad', 'a smidgen', etc. The phrase is generally used to objectify small clearances, dimensions, or distances.) In usage, the terms "hair", "cunt hair" and "red cunt hair" are related and each one is considered smaller and more precise than the preceding one ("hair" being largest and "red cunt hair" being smallest). It is thought to have originated in the late 1950s, as military slang. The phrase is associated with the traditionally male domains of military and engineering environments, and is used by some writers to evoke them. In Gravity's Rainbow, Thomas Pynchon invoked "a gnat's ass or red cunt hair" as images of very small units.
Other anatomically-based metaphorical measures
Winning a competition, such as a horse race, "by a whisker" (that is 'by a hair length') is a narrower margin of victory than winning "by a nose." An even narrower anatomically-based margin might be described in the idiom "by the skin of my teeth," which is typically applied to a narrow escape from impending disaster.