Hair's breadth

[2][3][4][5][6][7] This measurement is not precise because human hair varies in diameter, ranging anywhere from 17 μm to 181 μm [millionths of a metre][8] One nominal value often chosen is 75 micrometres (0.0030 in),[5] but this – like other measures based upon such highly variable natural objects, including the barleycorn[9] – is subject to a fair degree of imprecision.

The English "hair's breadth"[6] has a direct analogue in the formal Burmese system of Long Measure.

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).

[13][14] Carl Linnaeus had earlier recommended, in place of Joseph Pitton de Tournefort's geometric scale for botanical measurements, a scale starting with a "hair's breadth" (capillus) which was one 12th of a line (linea), one 6th of a (finger) nail (unguis), and likewise 144th of a thumb (pollex); which itself was equal to a (Parisian) inch.

"[18] Some German speakers similarly use “Muggeseggele,” literally “housefly’s scrotum,” as a small unit of measurement.