The standard pitch screw has a pitch approximately equal to the diameter. It is used on most horizontal installations and on inclines up to 20°. Half standard pitch screws may be used for inclines greater than 20°. Double- and triple-flight, variable- pitch, and stepped-diameter screws are available for moving difficult materials and controlling feed rate. Ribbon screws are used for wet or sticky substances. Special cut flight and ribbon screws are used for mixing, both singly and in connection with conveying