Hexagonal lattices (a–b planes, common c axis pointing upwards) of twin domain I (start domain, lattice points small circles, right-handed green unit cell a1, b1, c1), of the Σ7 twin-related domain II (small crosses, left-handed blue unit cell a2, b2, c2) and of the Σ7 coincidence lattice (large black points, right-handed red unit cell aT, bT, cT). The six alternative twin reflection planes m′(120), m′(120), m′(210), m′(410), m′(140) and m′(410) are indicated by dashed lines. The coordinate axes a2, b2, c2 of domain II (blue) are defined by the reflection plane m′(120). The right-handed yellow unit cell a3, b3, c3 of domain II is obtained from a1, b1, c1 by a clockwise rotation of φ = 120° + 2 arcsin [(1/2)(3/7)1/2] = 120° + 38.2° = 158.2° around the hexagonal axis (cf. Appendix C). This cell is commonly used in structure determinations.