This paper describes in a very easy and intelligible way, how the diffraction efficiencies of binary dielectric transmission gratings depend on the geometrical groove parameters and how a high efficiency can be obtained. The phenomenological explanation is based on the modal method. The mechanism of excitation of modes by the incident wave, their propagation constants and how they couple into the diffraction orders helps to understand the diffraction process of such gratings and enables a grating design without complicated numerical calculations.