Fig. shows a two-phase particle. The left part of the particle is of B 12 P 2 composition and contains polytypes, as evidenced by the elongated streaks passing through the spots in the corresponding FFT image (Fig. ). The right part contains a twice-twinned fragment of BP composition (the corresponding FFT image is shown in Fig. ). The mutual orientation of these two phases corresponds to the (003) B 12 P 2 ǁ (111) BP orientation relation. The lower part of BP fragment transforms into B 12 P 2 without a coherent boundary. The arrows denote the {111} BP planes. One of these planes separates B 12 P 2 and BP crystals, while the other two are twinning planes in BP crystal. Such a junction of twinning planes is generally denoted as Σ = 9 . In other words, Σ = 9 (second order twin) boundary is created when two Σ = 3 (first order twin) boundaries intersect.