Diffraction of parallel polarized electromagnetic waves from corrugated periodic perfectly conducting surfaces is analyzed by both a rigorous method and a method based on the Rayleigh hypothesis. The accuracy of the rigorous technique is established by comparing results against other theoretical and experimental results. Further results are then obtained and used as standards against which results based on the Rayleigh method are compared for accuracy. These comparisons indicate that the Rayleigh method gives reasonably accurate results for gratings with surface height to period ratios of up to

). This confirms that the Rayleigh method is also applicable to these nonanalytic surfaces and leads to accuracies that are higher than those normally assumed in the literature.