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About these units
Mile (mi)
The mile, equal to 5,280 feet, is a legacy of a long history of measurement stretching back to Roman times. Its ancestor, the Roman "mille passus" (thousand paces), established a rough distance of 1,480 meters, which evolved through medieval and early modern Europe into various regional miles. The English statute mile was standardized in 1593 and survives today in the United States, the United Kingdom (for roadways), and a few other territories. Despite being a non-metric unit, the mile remains deeply embedded in cultural and infrastructural systems. Road networks in the US are laid out with mile markers; athletic events include the famed "one-mile run"; and many people intuitively judge driving distances in miles. Aviation and international travel sometimes reference statute miles, though nautical miles dominate in navigation. Its long history and familiarity keep the mile relevant even in a metric world.
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.