Form II is closely related to Form I but has higher point group symmetry. Observed reflections that are common to both forms are highlighted in blue in Figure ), with the remaining extra reflections observed resulting from the higher symmetry Form II. A comparison of Form I and II is shown in Figure . Molecular arrangements within hydrogen-bonded layers are identical between forms (Figure and), with these layers in xanthine bearing a strong resemblance to the bonding found in hypoxanthine . In both forms, hydrogen-bonded layers are stacked.