Tables for
Volume A
Space-group symmetry
Edited by M. I. Aroyo

International Tables for Crystallography (2016). Vol. A, ch. 3.1, p. 710


P. M. de Wolffc

The net of lattice points in the plane of the reduced basis vectors a and b; OBAD is a primitive mesh. The actual choice of a and b depends on the position of the point P, which is the projection of the point [P_{0}] in the next layer (supposed to lie above the paper, thin dashed lines) closest to O. Hence, P is confined to the Voronoi domain (dashed hexagon) around O. For a given interlayer distance, P defines the complete lattice. In that sense, P and [P'] represent identical lattices; so do Q, [Q'] and [Q''], and also R and [R']. When P lies in a region marked [-c^{\rm II}] (hatched), the reduced type-II basis is formed by [{\bf a}^{\rm II}], [{\bf b}^{\rm II}] and [{\bf c} = -\overrightarrow{OP}_{0}]. Regions marked [c^{\rm I}] (cross-hatched) have the reduced type-I basis [{\bf a}^{\rm I},{\bf b}^{\rm I}] and [{\bf c} = + \overrightarrow{OP}_{0}]. Small circles in O, M etc. indicate twofold rotation points lying on the region borders (see text).