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About these units
Foot (ft)
A foot, equal to 12 inches, has been used for thousands of years and likely originated from the length of a human foot. Though early definitions varied, the modern standard foot—established internationally in 1959—provides a stable and consistent reference. The foot remains the foundation of architectural and construction measurements in the United States, influencing everything from lumber dimensions to ceiling heights and room layouts. Because the foot is intuitively relatable to human scale, people often estimate heights, furniture sizes, and walking distances in feet. In aviation, altitude is universally measured in feet worldwide, even in countries that otherwise use metric units—an unusual but deeply embedded global convention. The foot's persistence shows how cultural inertia can remain strong even when a unit predates modern scientific measurement.
Mil (mil)
A mil is an imperial-derived engineering unit equal to one-thousandth of an inch. It is used extensively in manufacturing, machining, and electrical engineering. Printed circuit board (PCB) trace widths, for instance, are often measured in mils because the unit provides a convenient scale for precision without resorting to decimals in inches. Because 1 mil equals 25.4 micrometers, it occupies a scale appropriate for tolerances in industrial production, coatings, film thickness, and gaskets. The mil is especially common in the United States, where certain engineering standards still rely on the imperial system. It provides an intuitive small-scale measurement for technicians accustomed to inches, helping avoid errors that might result from metric conversions.