Moganite, which is monoclinic at ambient temperature, undergoes a displacive transition to an orthorhombic phase at ≈570 K. Whereas the monoclinic phase may be considered as α-quartz that is Brazil twinned along {1 0 1¯ 1} at the unit-cell scale (cell-Twinning), the orthorhombic phase cannot be interpreted as a Brazil cell-Twin of β-quartz, in contrast to statements made in the literature. The shape of the oxygen tetrahedra in monoclinic moganite has been determined more reliably by density functional theory (DFT) calculations than by experiment: The differences between the various experimental results for the shape of the oxygen tetrahedra at ambient temperature are typically ten times larger than the differences between the DFT results. The DFT calculations suggest that the oxygen tetrahedra in moganite are very close in shape to the oxygen tetrahedra in α-quartz. Among the three DFT calculations considered, the most convincing results for the bond angles in moganite are obtained for the DMol³ code with functional PBE.