<p>The structural mechanisms of the transitions VI–VII(VIII), IV → VIII, IV → VI, XII → VIII, XII → VI, and XII → Ic are studied. It is shown that the transitions between more dense crystal ices IV, VI, VII(VIII), and XII usually proceed by small shifts of molecules with rearrangement of more than a half of the initial hydrogen bonds. In this case, only finite fragments of the initial phase are retained, which consist of a few molecules. At the same time, as was shown previously, the transitions between less dense crystal ices can proceed by retaining all hydrogen bonds in large (i.e., infinite at least in one dimension) fragments of the initial structure, with a possible change of the bond lengths and angles inside these fragments and rearrangement of hydrogen bonds only between these fragments. In this case, a large fraction (more than a half) of the initial bonds is retained. The difference in the structural mechanisms of transitions between more dense ices or between less dense ices is related to the degree of space filling in the ice structures. The structural mechanisms of the transitions IV → Ic and XII → Ic are very close to those of the transitions IV → VIII and XII → VIII, respectively, because the structures of ices IV and XII are intermediate between those of ices Ic and VII(VIII).</p>