An echelle grating (from French, échelle, meaning stairs or ladder) is a type of diffraction grating characterised by a relatively low groove density, but a groove shape which is optimized for use at a high incidence angle and therefore in high diffraction orders. Echelle gratings are, like other types of diffraction grating, used in spectrometers and similar instruments. They are most useful in cross-dispersed high resolution spectrographs, such as HARPS, PARAS (PRL Advanced Radial Velocity Abu Sky Search) and numerous other astronomical instruments.