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Miller indices (Miller indexes) One of the methods of notation devised to express the intercepts of crystal faces on crystallographic axes, proposed by W. H. Miller. The symbols h, k, and l represent whole numbers which are the reciprocals of intercepts along the a, b, and c (or x, y, and z) crystallographic axes respectively. A face which is parallel to a crystallographic axis is indicated by the symbol o. If a crystal face intercepts the negative end of an axis, a bar (negative sign) is placed above the appropriate symbol. A prism will have the general notation of 100 or 110, a pinacoid 001, and a pyramid 101 or 111. When the indices describe a crystal face they are conventionally written without brackets; when they describe crystal form, they are enclosed in brackets.