This standard is intended to describe a method called the "cross section"1 for the determination of the average diameter, that is, the arithmetic measurement of the actual diameters of the fibers that form a verranne yarn or of the filaments that form a yarn by silionne. These diameters should not be confused with the reference diameter (nominal diameter) which is defined as the one that corresponds to the actual average diameter of the filaments or discontinuous fibers, expressed in micrometers and rounded to the unit.