This letter explores the transmission resonances in metallic gratings with very narrow and deep slits, where the wavelengths of light can exceed the grating period. Using a transfer matrix formalism and a quasi-analytical modal expansion model, the authors demonstrate two primary mechanisms for light transfer: the excitation of coupled surface plasmon polaritons (SPPs) on both surfaces of the grating and the coupling of incident plane waves with waveguide resonances in the slits. These mechanisms can lead to near-perfect transmittance at specific resonant wavelengths. The study reveals that for deep gratings, transmission peaks appear at wavelengths slightly larger than the grating period, with the peaks moving to longer wavelengths as the grating height increases. The physical origin of these resonances is analyzed through a simplified modal method, showing that they are primarily due to the excitation of coupled SPPs and waveguide modes. The transmission behavior is sensitive to the angle of incidence and the width of the slits, with coupled SPPs showing strong dependence on the angle and waveguide modes being less sensitive. The authors suggest that these electromagnetic modes are responsible for the extraordinary optical transmission observed in hole arrays.This letter explores the transmission resonances in metallic gratings with very narrow and deep slits, where the wavelengths of light can exceed the grating period. Using a transfer matrix formalism and a quasi-analytical modal expansion model, the authors demonstrate two primary mechanisms for light transfer: the excitation of coupled surface plasmon polaritons (SPPs) on both surfaces of the grating and the coupling of incident plane waves with waveguide resonances in the slits. These mechanisms can lead to near-perfect transmittance at specific resonant wavelengths. The study reveals that for deep gratings, transmission peaks appear at wavelengths slightly larger than the grating period, with the peaks moving to longer wavelengths as the grating height increases. The physical origin of these resonances is analyzed through a simplified modal method, showing that they are primarily due to the excitation of coupled SPPs and waveguide modes. The transmission behavior is sensitive to the angle of incidence and the width of the slits, with coupled SPPs showing strong dependence on the angle and waveguide modes being less sensitive. The authors suggest that these electromagnetic modes are responsible for the extraordinary optical transmission observed in hole arrays.