We developed an intuitive analytical model for extraordinary optical transmission (EOT) through corrugated metallic films. In the framework of this model, EOT emerges from standing wave resonances in the different diffraction orders, and is an inherent property periodic structures, which does not require specific polarization dependent properties such as surface plasmons. Using the same underlying mechanism, the model correctly predicts the conditions for the EOT resonances in various geometrical configurations, in both TE and TM polarizations,and in the subwavelength and non-subwavelength regimes.