Firstly, diffraction patterns were summed over small local areas of the xanthine particle to improve the signal-to-noise from a given area and to highlight any streaking (e.g. Figure ), akin to that seen in 3D-ED data (e.g. Figure ). The streaking observed is consistent with that previously seen in guanine , amongst other systems . The streaking in reciprocal space is parallel to the direction defined by the normal to the stacked layers (as depicted in Figure ). However, there is no streaking through the reflections in the strong systematic row that passes through the origin, i.e. the (h0-h) reflections in Form I and (0k0) reflections in Form II. This is consistent with a rigid body transverse displacement of the layer with respect to the expected sequence, i.e. very much akin to a stacking fault, whose stacking fault vector is likely to be equal to A B C or D. The continuous streaking seen implies that these 'stacking faults' occur at near-random intervals.