By a Hair's Breadth
The Definition
By a very small distance or margin; a "close shave" or a narrow escape from disaster. It describes a situation where the difference between success and failure—or life and death—is so infinitesimal that it can be compared to the thickness of a single human hair.
The Deep Dive
The "junk knowledge" behind this phrase is that before the invention of the laser micrometer or the digital caliper, the human hair was one of the most consistent and universally available "precision" measuring tools in the world. While we use the phrase figuratively today, for centuries, it was a literal unit of measurement used by scientists, astronomers, and craftsmen.
The Universal Micrometer: While human hair varies in thickness based on genetics and health, the average diameter of a human hair is remarkably consistent at approximately 0.003 inches (75 microns). In the 17th and 18th centuries, this was as close to a "standard" microscopic unit as a person could get without specialized equipment.
The Cross-hair Connection: Early telescope makers and surveyors literally used human hair (and later, even finer spider silk) to create "cross-hairs" in their lenses. When an astronomer said they missed a star’s transit "by a hair," they weren't being poetic; they were describing a measurable gap in their field of vision.
The phrase reached its linguistic peak in the 16th century. It was famously utilized by William Shakespeare and appears in various early English Bible translations to describe precision in archery or narrow escapes from enemies. It represents the "junk" of the pre-industrial world—a measurement that was technically imprecise by modern standards, yet perfectly functional for a society that moved at a much slower pace.
Fast Facts
The "Spider Silk" Superiority: While the "hair's breadth" was the standard for general precision, the absolute "gold standard" for early optical instruments was spider silk, which is roughly 1/10th the thickness of a human hair.
The Biblical Archer: The Book of Judges describes 700 elite archers from the tribe of Benjamin who could "sling stones at a hair's breadth and not miss," establishing the phrase as a benchmark for superhuman accuracy.
The Metric Equivalent: In the modern "junk" world of engineering, a "hair" is often used as a slang term for one-thousandth of an inch (a "thou"), even though a real hair is usually three times that thickness.
References
Shakespeare, W. (1604). Othello. ("...of hair-breadth scapes i' the imminent deadly breach").
Hooke, R. (1665). Micrographia. (First detailed microscopic study of hair thickness).
Ammer, C. (2013). The Dictionary of Clichés. Skyhorse Publishing.
The Oxford English Dictionary. (2026). Hair (n.). Oxford University Press.