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About these units
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.
X-unit (X)
The X-unit is an extremely small length, approximately 1.002 × 10⁻¹³ meters, historically used to express X-ray and gamma-ray wavelengths. The unit was invented before modern standards for measuring electromagnetic wavelengths existed, allowing scientists to describe extremely short wavelengths without resorting to scientific notation. X-units were valuable in crystallography and atomic physics in the early 20th century, enabling precise description of spectral lines emitted by X-ray sources. Although modern practice has largely replaced the X-unit with the nanometer or picometer, it continues to appear in historical literature. The unit's existence highlights how scientific progress shapes measuring conventions. Once essential, the X-unit now serves as a bridge to the history of early atomic research.