For a fully disordered solid (i.e. no crystal nuclei present) we hypothesise that no crystallisation is possible at temperatures below T gβ as the molecular interactions are confined by localised energy minima. It is only at temperatures above T gβ that largescale conformational changes can take place that provide sufficient molecular mobility for crystal nuclei to form. However, whilst crystallisation will be possible at such temperatures it will be a very slow process for most materials as there is not sufficient free volume available in the solid for translational or rotational motions to take place. As the temperature increases further the density of the solid decreases as the amount of free volume in-creases until at T gα there is sufficient mobility and free volume available for the glass transition to take place. Whether or not a material will fully crystallise at temperatures T gβ < T < T gα , or indeed at all, will depend on the molecular size, flexibility and intermolecular bond strength of the molecule.