Conformational polymorphism occurs when a compound crystallises in two polymorphs differing only in the relative orientations of flexible groups ( e.g. Ritonavir).[cite]10.1039/D1SC06074K[/cite] At the Beilstein conference, Ian Bruno mentioned another type; tautomeric polymorphism , where a compound can crystallise in two forms differing in the position of acidic protons. Here I explore three such examples.