We present a method to characterize subnanometric layers based on grazing incidence X-ray reflectometry. For this purpose, we propose to use a "Fabry-Perot" type multilayer structure in order to improve the sensitivity of the measurement to the layer thickness. For our study, this structure consists of a thin layer of scandium inserted between two periodic chromium (Cr)/scandium (Sc) multilayers. We describe the principle and estimate the sensitivity of the method by simulation. Experiments were performed on two optimized Fabry-Perot structures with 0.6 and 1.2 nm Sc layer thicknesses using a laboratory grazing incidence reflectometer at 8.048 keV (Cu K alpha radiation). Fitting of experimental data allows determining the Sc layer thickness. Finally, the structural parameters used in the fit were confirmed by measurements at 3 keV on the hard X-ray branch of the synchrotron SOLEIL Metrology and Tests beamline. (C) 2014 Elsevier B.V. All rights reserved.