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The French scale or French gauge system is commonly used to measure the size of a catheter. It is most often abbreviated as Fr, but can often be seen abbreviated as Fg, Ga, FR or F. It may also be abbreviated as CH or Ch (for Charrière, its inventor).
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.