Vectors C and D are defined to minimise the magnitude of the vectors (further detail on how these vectors are defined is shown in Figure .2). Movement between adjacent layers may be described by one of these four vectors. As shown in Figure ) and iv), in Form I, the layer stacking follows repeated application of vectors A and B. Layer positions do not repeat after any set number of layers, consistent with the monoclinic symmetry of the cell. In Form II, the stacking of these layers follows the sequence A B C D, with atom co-ordinates repeating every four layers.