A transition in which the space-group symmetry of the lower symmetry phase is a subgroup of that of the higher symmetry phase. Example: The transition of the low-temperature polymorph of quartz characterized by space-group symmetry \(P3_{1}2\) (trigonal), to the high temperature polymorph of quartz with space-group symmetry, \(P6_{2}22\) (hexagonal). Synonymous with @S06212@ (note: this term, strictly speaking, is no longer in use).