Reciprocal tetragonal lattices (hk0 lattice planes) of twin domain I (start domain, lattice points small circles) and of the Σ5 twin-related domain II (small crosses). The reciprocal lattice of the (direct-space) Σ5 coincidence lattice is represented by the grid of small squares. The unit cells, their handedness and their colours correspond to those of the direct lattices in Fig. 4. In the square formed by the four reciprocal coincidence points 000, 20, 310, 120 (in terms of a1*, b1*) or 000, 500, 550, 050 (in terms of aT*, bT*) there are four `single' points of twin domains I and II each, one `coincident' point 000 and, with reference to aT*, bT*, 16 `extinct' reciprocal points (cf. Table 1). These strange `non-space-group extinctions' are characteristic of the Σ5 twin law.