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Ruled plane diffraction gratings are produced by physically forming grooves on a reflective surface with a ruling engine. The distance between adjacent grooves and the angle they form with the substrate affect both the dispersion and efficiency of the grating. The first diffraction gratings made for commercial use were mechanically ruled. It was manufactured by burnishing grooves individually with a diamond tool against a thin coating of evaporated metal applied to a plane or concave surface. Today, several types of ruling engines are applied to produce gratings with low Rowland ghosts, high resolving power, and high efficiency uniformity. Ruled gratings can be blazed for specific wave lengths and are generally desired because of their high efficiency. They are often used in systems requiring high resolution, and comprise the majority of diffraction gratings used in spectroscopic instrumentation.