This letter presents a study on transmission resonances in metallic gratings with very narrow and deep slits. Using a transfer matrix formalism and a quasi-analytical model based on modal expansion, the authors show that these gratings can exhibit transmission resonances for wavelengths larger than the grating period. Two mechanisms are identified: 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. Both mechanisms can lead to almost perfect transmittance at specific resonances. The study also explores the physical origin of these transmission resonances, showing that they are linked to the coupling of incident light with surface electromagnetic modes of the grating. The zero-order transmittance spectrum is governed by the behavior of a denominator D, which determines the transmission peaks. The authors also analyze the photonic band structure of these surface plasmons, showing that transmission resonances are mainly due to the excitation of two types of electromagnetic modes: coupled SPPs for wavelengths close to the grating period and waveguide resonances for wavelengths much larger than the period. The transmission resonances are analyzed as a function of the width of the slits, showing that for coupled SPPs, a minimum slit width is needed for effective coupling, while for waveguide resonances, even very narrow slits can lead to high transmittance. The transmission process for coupled SPPs is sensitive to the presence of a substrate, while for waveguide resonances, it is not. The authors conclude that these transmission resonances are responsible for the extraordinary optical transmission observed in hole arrays. The study provides a detailed analysis of the transmission properties of metallic gratings with very narrow slits, using both numerical simulations and analytical methods.This letter presents a study on transmission resonances in metallic gratings with very narrow and deep slits. Using a transfer matrix formalism and a quasi-analytical model based on modal expansion, the authors show that these gratings can exhibit transmission resonances for wavelengths larger than the grating period. Two mechanisms are identified: 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. Both mechanisms can lead to almost perfect transmittance at specific resonances. The study also explores the physical origin of these transmission resonances, showing that they are linked to the coupling of incident light with surface electromagnetic modes of the grating. The zero-order transmittance spectrum is governed by the behavior of a denominator D, which determines the transmission peaks. The authors also analyze the photonic band structure of these surface plasmons, showing that transmission resonances are mainly due to the excitation of two types of electromagnetic modes: coupled SPPs for wavelengths close to the grating period and waveguide resonances for wavelengths much larger than the period. The transmission resonances are analyzed as a function of the width of the slits, showing that for coupled SPPs, a minimum slit width is needed for effective coupling, while for waveguide resonances, even very narrow slits can lead to high transmittance. The transmission process for coupled SPPs is sensitive to the presence of a substrate, while for waveguide resonances, it is not. The authors conclude that these transmission resonances are responsible for the extraordinary optical transmission observed in hole arrays. The study provides a detailed analysis of the transmission properties of metallic gratings with very narrow slits, using both numerical simulations and analytical methods.