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 P 3 1 2 (trigonal), to the high temperature polymorph of quartz with space-group symmetry, P 6 2 2 2 (hexagonal). Synonymous with symmetry-breaking transition (note: this term, strictly speaking, is no longer in use).