In their bulk forms, dispersion corrected density functional theory simulations suggest the polymorphic stability to follow as Form B > Form C > Form A (ESI Section 7). Notably, the relative stability of Forms B and C derive from entropic contributions, which can be easily affected by small changes in temperature. We however note that our simulations suggest that no re-ordering of the polymorphic phases occurs with temperature. Instead, the ability to reproducibly and selectively obtain each polymorphic form can be rationalized by considering crystallite sizes generated by milling under different conditions. We have shown previously that for nanocrystalline materials, polymorph stabilities depend not only on internal lattice energy, but also on crystal size and surface solvation effects. Ball mill grinding breaks crystals down to nanometre length scales, with a dramatic increase in their surface to volume ratio. We previously performed polymorph interconversion turnover experiments between Form A and Form B, showing that the crystal size (Scherrer size) of Form A was consistently smaller (around 40 nm) as compared to Form B (60-79 nm). Once crystals reach this nano scale, a significant proportion of their molecules are located on the surface of the crystal, with a lower number of stabilizing intermolecular interactions as compared with molecules in the bulk. We believe that the presence of additives (e.g. liquid) acts to stabilize these surfaces, thereby altering the relative stability of nanocrystals, and therefore with the potential to cause a polymorph conversion. These fundamental concepts are universal and apply to all chemistries. Estimates via the Scherrer equation revealed that at least one of the two polymorphs involved in the previous two-polymorph turnover experiments that we studied had a crystal size smaller than 100nm. Analogously, Form A and Form C have crystal sizes in the order of tens of nanometers. Moreover, the crystal sizes of all three polymorphs (Form A, Form B and Form C) are also highly reproducible, independently of whether they are obtained in the clockwise or anticlockwise turnover cycle. Consistently, Form A has the smallest crystals (50 nm) while Form B crystals are the largest (140 nm) (Figure ) and d)). This experimental observation hints at a difference of the bulk:surface energy balance across the three polymorphs. Moreover, the liquid used under LAG conditions must interact with crystal surfaces affecting surface stabilities as a consequence. The polymorph stability order is thus influenced by the solvent nature and the concentration used as well as the crystal size and morphology. Conceptually, each polymorph can be thought of as representing a local thermodynamic well, accessible under the specific set of milling conditions. This is demonstrated by the fact that these polymorph conversions can be reversed by changing the milling conditions. Moreover, our computational studies (ESI Section 7) show an inverse correlation between the polymorph lattice energies and the experimental crystal size obtained by the ball mill grinding. This strongly suggest that different polymorphic forms must grow to different sizes before 'bulk' properties dominate their stability, consistent with earlier findings. A thorough understanding of nano-scale stability will clearly be critical for determining the outcome of ball milling transformations. It is clear the nanoscale crystal properties are responsible for determining the result of polymorphic transformations under ball milling conditions. However, it is not yet clear whether these polymorph interconversions: (a) occur via some reconstructive mechanism, dominated by nucleation and growth; or (b) are of a single crystal to single crystal transformation (see Figure ). In the latter case, the crystallites of one polymorph approach a threshold crystal size under the specific milling conditions beyond which a phase transformation of displacive nature occurs towards a more stable polymorph. In the former case, the polymorph is dictated by which nuclei are stabilised (or destabilised) under the given milling conditions. It is subsequently this nucleus that grows to the size that we observe at milling equilibrium. [23d, 23g, 26] From the crystal perspective, this size is dictated by the balance between internal enthalpic stabilisation and the destabilising effects of a growing surface. However, under ball milling conditions crystal growth and crystal breaking by the ball milling presumably play a dominant role in the final crystal size. Our previous work has suggested that, within a small range (ca. 15-30 Hz), milling frequency does not alter the crystal size at milling equilibrium. Although we might expect in principle the equilibrium to be sensitive to both milling frequency and energy, we suggest that the parameter range accessible within a conventional ball mill is too narrow to make this effect observable. This suggests that milling equilibria obtained in this way are insensitive to small changes in milling conditions, making them robust and transferable. This is will be important for the translation of academic research into industrial settings.