A hair's breadth, or the width of human hair, is used as an informal unit of length.
Until the middle of the 20th century, the highest resolution of measurement was considered to be the same order of magnitude, around 10−5metres, as the diameter of a human hair. A "hair's breadth" was, and still is, informally, a very small measurement.
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).
A red cunt hair (also RCH,red pussy hair, and red one) 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.
^ abCrook, John; Osmaston, Henry (1994). "Weights and Measures". Himalayan Buddhist Villages. Delhi: Shri Jainendra Press. p. 133. ISBN978-0862923860.
^ abJohnson, Cuthbert William (1842). "Weights and Measures". The farmer's encyclopædia, and dictionary of rural affairs. London: Longman, Brown, Green, & Longman. p. 1257.
^Boaz, James (1823-03-21). "On a fixed Unit of Measure". In Tilloch, Alexander; Taylor, Richard. Philosophical Magazine61. London: Richard Taylor. p. 267.
^Latter, Thomas (1991). "Measures". A Grammar of the Language of Burmah (republished ed.). Asian Educational Services. p. 167. ISBN9788120606937.
^Carey, Felix (1814). "Of Weights &c.". A grammar of the Burman language. Mission Press. p. 209.
^Maunder, Samuel (1855). "Measures of Length". Treasury of Knowledge and Library of Reference. New York: J. W. Bell. p. 12.
^Lindley, John (1839). "Glossology". An introduction to botany (3rd ed.). London: Longman, Orme, Brown, Green, & Longmans. p. 474.
^Withering, William (1818). "Botanical Terms". An Arrangement of British Plants1 (6th ed.). London: Longman & Co., Robert Scholey, et al. p. 69.
^Spelvin, Georgina (2008). The Devil Made Me Do It. Lulu.com. p. 110. ISBN0-615-19907-0. "'Oh, just another RCH,' replies the perfectionist. 'What's an RCH?' I naively inquire. The crew replies in chorus, 'Red Cunt Hair.' I was learning a wealth of technical jargon."
^Michaelis, David (1983). The best of friends: profiles of extraordinary friendships. Morrow. p. 231. ISBN0-688-01558-1. "a mortar round came in and missed you by an R.C.H. [an R.C.H. being survivors' lingo for something very thin—a Red Cunt Hair] and that sucker missed you by one of those."
^Dalzell, Tom (2008). The Concise New Partridge Dictionary of Slang and Unconventional English. Routledge. p. 535. ISBN0-415-21259-6. "red one [...] a very short distance. A euphemized abbreviation of Red Cunt Hair. Red pussy hair — a very short distance. Slightly less offensive than Red Cunt Hair."
^Partridge, Eric; Dalzell, Tom; Victor, Terry (2006). The New Partridge Dictionary of Slang and Unconventional English (9 ed.). Taylor & Francis. p. 1596. ISBN0-415-25938-X. "RCH (noun) a tiny notional unit of measure (US) An abbreviation of Red Cunt Hair, perceived as a smaller unit even than a simple Cunt Hair"
^Dorson, Richard Mercer (1986). Handbook of American Folklore. Indiana University Press. p. 123. ISBN0-253-20373-2. "Small measurements are frequently referred to as 'just a cunt hair,' or, if more accuracy is preferred, as a 'red cunt hair' (no other color is ever used)."
^Partridge, Eric; Dalzell, Tom; Victor, Terry (2006). The New Partridge Dictionary of Slang and Unconventional English. Taylor & Francis. p. 1601. ISBN0-415-25938-X.
^Dickson, Paul (1994). War Slang: Fighting Words and Phrases of Americans from the Civil War to the Gulf War. University of Michigan. p. 286. ISBN0-671-75022-4. "Red cunt hair. A very small distance (as in, 'That is one red cunt hair out of alignment.')."
^Hales, John (2005). Shooting Polaris. University of Missouri Press. p. 45. ISBN0-8262-1616-1. "Bump it a little west now, a hair more, a cunt hair now ... a red cunt hair ... good."