Crystallography
In crystallography, a screw axis is a symmetry operation describing how a combination of rotation about an axis and a translation parallel to that axis leaves a crystal unchanged.
Screw axes are noted by a number, n, where the angle of rotation is 360°/n. 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 1/2 of the lattice vector. 31 is a 120° (threefold) rotation followed by a translation of 1/3 of the lattice vector. The possible screw axes are 21, 31, 41, 42, 61, 62, and 63, and the enantiomorphous 32, 43, 64, and 65.
Read more about this topic: Screw Axis