There are considerable similarities in their respective crystal structures, which permit the observed intergrowth behaviour. Figure shows the projections of the two polymorphs along the a, b and c axis. The structures are polytypes 8 , having consistent layers lying in the bc planes. For the two forms, the difference is seen in the arrangement of these layers along the a axis. The relationship between the structures can be viewed as a shift of neighbouring layers by half a unitcell length along the c axis (as can be seen in the a axis projection), or alternatively as a reflection of every second layer perpendicular to the b axis. Given the observed intergrowth behaviour, it is evident that the two arrangements must have closely comparable energies.