Twin domains are related by a symmetry operator , defined as the twin law. To preserve the strong intra-planar hydrogen bonding, any twinning mechanisms in xanthine likely involve interlayer displacement which disrupts only the weaker van der Waals' bonding . To determine whether the xanthine lattices were indeed twinned (as opposed to just two misoriented crystals) the orientation matrices were compared. This revealed an orientation relationship which can be described using either a rotation, in this case about [101], or a reflection about (101 ̅ ); confirming a twinning mechanism and a twin law in xanthine. This analysis and the notation to describe this twin law is given in greater depth in S3, following deformation twin notation. It is not possible to distinguish between the rotation or reflection operation in projection given that Form I xanthine (with P21/c space group) has a centrosymmetric structure. The rotation twin about [101] preserves intra-layer molecular arrangements between twin domains (Figure ) and acts to flip the layers. Since the composition surface (the surface along which the lattice points between domains are shared) are parallel to each other, this is a polysynthetic twin and may lend itself to multiple twinning . A structure solution for the orthorhombic phase was successfully achieved using ab initio Dual Space methods implemented in SHELXD, indicating the P212121 space group (Z'=4, Z=16) with a 79% completeness. All non-hydrogen atoms were found in the initial structure solution.