X unit (source code)
= X unit
{c}
{title2=$\mathrm{xu}$}
{wiki}
The historical <X unit> was a practical <X-ray> <wavelength> unit, approximately $10^{-13}\,\mathrm m$. Its scale used an assigned calcite spacing; relative <wavelength> measurements could be more precise than absolute determinations. Variation between crystal specimens led to calibration using reference <spectral lines>. An <X unit> value therefore cannot simply be read as an exact multiple of $10^{-13}\,\mathrm m$.<Nomenclature System for X-ray Spectroscopy prepared by R. Jenkins and others>[IUPAC, Nomenclature system for X-ray spectroscopy, §5.1 Units, printed p. 745]{ref}
An angular-scale check determines how an indicated rotation corresponds to an angle. A reference line transfers a <wavelength> scale. An absolute determination ties that scale to an independently established <length> standard. These are different calibration tasks. Repeating measurements can reduce random scatter without correcting the common scale: if every inferred $d$ is too large by a chosen $0.1\%$, all inferred absolute <wavelengths> are too large by $0.1\%$, while ratios using that same $d$ are unchanged.
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