The Bragg condition λ = 2d sin θ is presented as a necessary condition for perfect (100%) blazing of infinite, perfectly conducting diffraction gratings that produce only a single diffracted order, n = −1. As an illustration, the rectangular-profile grating is analyzed. Design blaze curves, showing the required depth as a function of angle of incidence, for each polarization, are obtained. Intersections of corresponding blaze curves for the two polarizations yield parameters of gratings perfectly blazed simultaneously in both polarizations. The angle of incidence at which this is achieved depends on the shape of the grating. Gratings so designed have a theoretical 100% efficiency at the design frequency, and above 90% over a bandwidth of more than 10%.