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.
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).
Winning a competition, such as a horse race, "by a whisker" (that is 'by a hair') is a narrower margin of victory than winning "by a nose." An even narrower margin might be described in the idiom "by the skin of my teeth," which is typically applied to a narrow escape from impending disaster.
Red cunt hair
|Look up by a hair's breadth, hair, or red cunt hair in Wiktionary, the free dictionary.|
A subset of "hairs breadth" is the red cunt hair (also RCH,[A][B] red pussy hair, and red one[C] which is a figure of speech used to represent very small widths.[D] [E] 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.[F] 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).[G] 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.
- 15 µm — width of silk fibre
- 17 µm — minimum width of a strand of human hair
See 10 micrometres.
- Beard-second 
- List of humorous units of measurement
- List of unusual units of measurement
- "'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."
- "... 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."
- "...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."
- "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"
- "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)."
- "Red cunt hair. A very small distance (as in, 'That is one red cunt hair out of alignment.'"
- "Bump it a little west now, a hair more, a cunt hair now ... a red cunt hair ... good."
- Roughly a housefly's 'package,' the smallest Swabian unit of measurement. An entomologist at the Naturkundemuseum in Stuttgart gave an average length of 0.22 millimetres (0.0087 in). This is roughly equivalent to the English idiomatic phrase "as small as the hairs on a gnat's bollock."
- "Hair’s breadth (hare’s breath)". The Grammarist. Retrieved January 27, 2015.
- Hairs breadth. Oxford English Dictionary. Retrieved January 28, 2015.
- "Hairs breadth". Macmillan English Dictionary. Retrieved January 27, 2015.
- "Hairs breadth". Cambridge Dictionary. Retrieved January 27, 2015.
- Smith 2002, p. 253.
- Crook & Osmaston 1994, p. 133.
- Johnson 1842, pp. 1257.
- Boaz 1823, p. 267.
- Latter 1991, pp. 167.
- Carey 1814, p. 209.
- Maunder 1855, p. 12.
- Lindley 1839, p. 474.
- Withering 1818, p. 69.
- "Win by a nose". The American Heritage® Idioms Dictionary. Houghton Mifflin Company/Dictionary.com. 2002. Retrieved January 27, 2015.
- By the skin of one's teeth Wiktionary.
- "The meaning and origin of the expression: By the skin of your teeth". The phrase finder. Retrieved January 28, 2015.
- Spelvin 2008, p. 110.
- Michaelis 1983, p. 23.
- Partridge, Dalzell & Victor 2008, p. 535.
- Partridge, Dalzell & Victor 2008, p. 1596.
- Dorson 1986, p. 123.
- Partridge, Dalzell & Victor 2008, p. 1601.
- Johnson 1995.
- Dickson 1994, p. 286.
- Hales 2005, p. 45.
- Morton 2003, p. 134.
- Raudaskoski 1997.
- Ley, Brian; Elert, Glenn, Editor (1999). Width of a Human Hair. The Physics Factbook: An Encyclopedia of Scientific Essays. Retrieved October 26, 2014.
- Shimmin, Graeme. "What are the most obscure least useful or craziest units of measurement?". Retrieved October 26, 2014.
- Sellner, Jan (9 March 2009). "Schönstes schwäbisches Wort: Großer Vorsprung für Schwabens kleinste Einheit". Stuttgarter Nachrichten (in German). Retrieved 13 August 2013.
- Petersen, Michael (3 November 2008). "Schwäbisch für Reigschmeckte: 'Was ist denn bitte schön ein Muggaseggel?'". Stuttgarter Zeitung (in German). Retrieved 13 August 2013.
- Petershagen, Henning. "Schwäbisch auf Anfrage: 1 Muggaseggel = 0,22 Millimeter". Süddeutsche Heimat Zeitung (in German). - Online as jpg
- Henning Petershagen, 1 Muggaseggl = 0,22 Millimeter.
- Boaz, James; Tilloch, Alexander, Editor; Taylor, Richard, Editor (1823-03-21). "On a fixed Unit of Measure". Philosophical Magazine 61. London: Richard Taylor. p. 267.
- Carey, Felix (1814). "Of Weights &c.". A grammar of the Burman language. Mission Press/Google books. p. 209.
- Crook, John; Osmaston, Henry (1994). "Weights and Measures". Himalayan Buddhist Villages. Delhi: Shri Jainendra Press. p. 133. ISBN 978-0862923860.
- Dickson, Paul (1994). War Slang: Fighting Words and Phrases of Americans from the Civil War to the Gulf War. Ann Arbor: University of Michigan Press. p. 286. ISBN 0-671-75022-4.
- Dorson, Richard Mercer (1986). Handbook of American Folklore. Bloomington, Indiana: Indiana University Press. p. 123. ISBN 0-253-20373-2.
- Hales, John (2005). Shooting Polaris a personal survey in the American West. Columbia, Missouri: University of Missouri Press. p. 45. ISBN 0-8262-1616-1.
- Johnson, Cuthbert William (1842). "Weights and Measures". The farmer's encyclopædia, and dictionary of rural affairs. London: Longman, Orme, Brown, Green, & Longmans/Internet Archive. p. 1257.
- Johnson, Sterling (1995). English as a Second f*cking Language. New York: Saint Martin's Press, St. Martin's Griffin. ISBN 978-0-312-14329-9.
- Latter, Thomas (1991). "Measures". A Grammar of the Language of Burmah (republished ed.). Asian Educational Services. p. 167. ISBN 9788120606937.
- Lindley, John (1839). "Glossology". An introduction to botany (3rd ed.). London: Longman, Orme, Brown, Green, & Longmans. p. 474.
- Maunder, Samuel (1855). "Measures of Length". Treasury of Knowledge and Library of Reference. New York: J. W. Bell. p. 12.
- Michaelis, David (1983). The best of friends: profiles of extraordinary friendships (Print). New York: Morrow. p. 231. ISBN 0-688-01558-1.
- Morton, Mark S. (2003). The lover's tongue a merry romp through the language of love and sex (Print). Toronto Ontario: Insomniac Press. p. 134. ISBN 1-894663-51-9.
- Partridge, Eric; Dalzell, Tom; Victor, Terry (2008). The Concise New Partridge Dictionary of Slang and Unconventional English (Print). London New York: Routledge. pp. 535, 1596 & 1601. ISBN 0-415-21259-6.
- Raudaskoski, Heikki (January 1997). 'The Feathery Rilke Mustaches and Porky Pig Tattoo on Stomach': High and Low Pressures in Gravity's Rainbow. Postmodern Culture 7 (2). Retrieved January 20, 2015.
- Smith, Graham T. (2002). Industrial metrology. Springer. p. 253. ISBN 9781852335076.
- Spelvin, Georgina (2008). The Devil Made Me Do It (Print). Los Angeles, California: Lulu.com, Little Red Hen Books. p. 110. ISBN 0-615-19907-0.
- Withering, William (1818). "Botanical Terms". An Arrangement of British Plants 1 (6th ed.). London: Longman & Co., Robert Scholey, et al. p. 69.