The nature of different degrees of planar disorder in xanthine stems from stacking variation between its hydrogen-bonded layers. As a small organic molecule with an archetypal purine ring structure, insights derived here may also be applicable to a range of other molecular crystal systems. Cytosine 49 , thymine , guanine, adenine, uracil , eniluracil , caffeine, and theophylline are key examples amongst a large class of inter-related molecules which have layered structures that may exhibit similar behaviour. We discuss and compare interlayer stacking in some of these systems.