The direction of layer stacking is described by the normal to the (101 ̅ ) planes in Form I and by the normal to the (010) planes in Form II. The difference between Form I and II xanthine lies in their inter-layer stacking sequence. Therefore, Form II can be described as a polytype of Form I. Different polytypes may arise when multiple stacking arrangements lie within a shallow energy landscape and there are multiple energetically-favourable configurations .