This document specifies the substrate conditions and testing of the modulation period (including the principles for low-angle X-ray methods, the requirements of the coatings, the requirements for X-ray measuring apparatus, the calibration of apparatus and samples, and the testing conditions and calculation process) of nano-multilayer coatings by low-angle X-ray methods including X-ray reflectivity (XRR) and glancing incident X-ray diffraction (GIXRD).