We further investigated the energetic driving force for the observed A-site order using DFT calculations of K{Ni[Bi(SCN ]. The stability of each cation order thus depends on the how easily the parent framework can deform to accommodate a given order. ]} revealed that it also crystallises as a double double vacancy-ordered perovskite which, like 1 and 2, has a structure derived from an ReO 3 -type parent by introducing cations into half of the pseudocubic cages [Fig. ]. However, 3 has a more complex structure than 1 and 2 and its unit cell is a (2 × 6 × 4) monoclinic P2/n supercell of the Pm 3m aristotype (i.e. is 12 times larger than the structures 1 and 2), due to an unusual ordering of the MeNH + 3 cations and complex octahedral tilting. The complexity of the order, together with the high metric pseudosymmetry ( c 3 = 1.004 a, β = 90.2 • ), means that 3 has a high propensity for twinning. We found that twinning is common around both the [001] and [110] lattice directions, which made structure solution and refinement challenging. Using SCXD data collected on a small crystal (65 × 55 × 22 µm) which only had a minor twin component (<20%, 180 • twin around the [001] direction), we were able to produce a stable refinement for the structure.