A hair's breadth, or the width of human

hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and f ...

, is used as an informal

unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical presentation Music * ''Unit'' (alb ...

of a very short

length Length is a measure of distance. In the International System of Quantities, length is a quantity with dimension distance. In most systems of measurement a base unit for length is chosen, from which all other units are derived. In the Interna ...

. It connotes "a very small margin" or the narrowest degree in many contexts.

Definitions

This measurement is not precise because human hair varies in diameter, ranging anywhere from 17 μm to 181 μm illionths of a metreOne nominal value often chosen is 75 μm, but this – like other measures based upon such highly variable 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 ''Sesamum'' is a leguminous crop and genus of about 20 species in the flowering plant family Pedaliaceae. The plants are annual or perennial herbs with edible seeds. The best-known member of the genus is sesame, ''Sesamum indicum'' (syn. ''Ses ...

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 Measuring tape with inches The inch (symbol: in or ″) is a unit of length in the British imperial and the United States customary systems of measurement. It is equal to yard or of a foot. Derived from the Roman uncia ("twelfth") ...

(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 or ~176 μm (a line itself being one 12th of an inch).

Other body part measurements

Winning a competition, such as a

horse race Horse racing is an equestrian performance sport, typically involving two or more horses ridden by jockeys (or sometimes driven without riders) over a set distance for competition. It is one of the most ancient of all sports, as its basic p ...

, "by a whisker" (a short beard hair) 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. This is roughly analogous to the idiomatic phrase "as small as the hairs on a gnat's

bollock ''Bollocks'' () is a word of Middle English origin, meaning "testicles". The word is often used figuratively in British English and Hiberno-English in a multitude of negative ways; it most commonly appears as a noun meaning "rubbish" or "nonsen ...

." German speakers similarly use “

Muggeseggele A ''Muggeseggele'' or ''Muckenseckel'' is a humorous Alemannic German idiom used in Swabia to designate a nonspecific very small length or amount of something; it refers to a housefly's scrotum. It has been called the smallest Swabian unit of m ...

,” literally “housefly’s scrotum,” as a small unit of measurement.

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