on C faces only R — Rhombohedral Screw axes The screw axis is noted by a number, n, where the angle of rotation is . The degree of translation is then added as a subscript showing how far along the axis the translation is, as a portion of the parallel lattice vector. For example, 21 is a 180° (twofold) rotation followed by a translation of of the lattice vector. 31 is a 120° (threefold) rotation followed by a translation of of the lattice vector. The possible screw axes are: 21, 31, 32, 41, 42, 43, 61, 62, 63, 64, and 65. There are 4 enantiomorphic pairs of axes: (31 — 32), (41 — 43), (61 — 65), and (62 — 64). This enantiomorphism results in 11 pairs of enantiomorphic space groups, namely Glide planes Glide planes are noted by a, b, or c depending on which axis the glide is along. There is also the n glide, which is a glide along the half of a diagonal of a face, and the d glide, which is along a quarter of either a face or space diagonal of the unit cell. The d glide is often called the diamond glide plane as it features in the diamond structure. a, b, or c glide translation along half the lattice vector of this face. n glide translation along half a face diagonal. d glide planes with translation along a quarter of a face diagonal or of a space diagonal. e two glides with the same glide plane and translation along two (different) half-lattice vectors. References External links Decoding the Hermann-Maguin Notation - An introduction into the Hermann-Maguin notation for beginners. Crystallography Chemical nomenclature