**Figure 3**
The effect of twinning by a twofold rotation about **a** on the diffraction pattern of a monoclinic crystal with β = 90°. Only the *h*0*l* zone is illustrated; the space group is *P*2_{1}/*c*. (*a*) *h*0*l* zones from a single-crystal. This could represent the diffraction pattern from one domain of a twinned crystal. (*b*) This is the same pattern as shown in (*a*), but rotated about **a*** (or *h* axis) (which is coincident with the *a* axis of the direct cell). This figure represents the diffraction pattern from the second domain of a twinned crystal. (*c*) Superposition of (*a*) and (*b*) simulating a twin with a domain scale factor of 0.5; that is, both domains are present in equal amounts. (*d*) Superposition of (*a*) and (*b*) simulating a twin with a domain scale factor of 0.2: the crystal consists on 80% of one domain (*a*) and 20% of the other (*b*). The values of |*E*^{2} − 1| for each figure are (*a*) and (*b*) 1.015, (*c*) 0.674, (*d*) 0.743. The ideal (untwinned) value of |*E*^{2} − 1| for this centric crystal structure is 0.97, meaning that its diffraction pattern characterized by the presence of both strong and weak reflections; intensities are more evenly distributed in acentric distributions, where |*E*^{2} − 1| has an ideal value of 0.74. |