This higher-order structural variation of a polymorph is commonly known as polytypism, whereby differences in crystal structures are solely caused by the difference in the stacking sequence of layers. However, although the nature of hydrogen bonding interactions will likely remain the same, the deviation of each molecule from the mean plane of the layer will not necessarily be preserved as is the case in guanine (Figure .5 c-d). Therefore, we propose to name our monoclinic polymorph of xanthine presented in this work the Form I polymorph.