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The x unit (symbol xu) is a unit of length approximately equal to 0.1 pm (10−13 m). It is used to quote the wavelength of X-rays and gamma rays.
The spat (symbol S) is also an obsolete unit of distance used in astronomy. It is equal to 1000000000 kilometres (1 Tm or 1012 m). It is about 6.6846 astronomical units (AU), 1.057×10−4 light-years or3.240×10−5 parsecs.