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The transmission resonance characteristics of a gold double-layered compound grating (DCG) are investigated numerically using the rigorous coupled-wave analysis (RCWA) method. The DCG is assumed to comprise two identical single-layered gratings separated by an air gap with a thickness of 0 ~ 250 mm. Furthermore, each grating is assumed to comprise two slits with different widths (w) within each period. The physical origins of the transmission features of the DCG structures are examined using the finite-difference time-domain (FDTD) method. The RCWA results show that the optical transmission spectra of the considered DCG structures can be tailored over a wide range of wavelengths via a suitable adjustment of the air gap thickness. Moreover, it is shown that, for values of the air gap thickness greater than 100 nm, the transmission spectrum of the DCG structure converges toward that of a single-layered compound grating (SCG). Overall, the results presented in this paper show that the optical transmission properties of a metallic DCG are highly sensitive to the thickness of the air gap between the two grating structures. As such, DCG structures are ideally suited to the realization of proximity sensors and other photonic-based optical devices.