In historical linguistics, phonological change is any sound change that alters the distribution of phonemes in a language. In other words, a language develops a new system of oppositions among its phonemes. Old contrasts may disappear, new ones may emerge, or they may simply be rearranged. Sound change may be an impetus for changes in the phonological structures of a language (and likewise, phonological change may sway the process of sound change). One process of phonological change is rephonemicization, in which the distribution of phonemes changes by either addition of new phonemes or a reorganization of existing phonemes. Mergers and splits are types of rephonemicization and are discussed further below. Types In a typological scheme first systematized by Henry M. Hoenigswald in 1965, a historical sound law can only affect a phonological system in one of three ways: Conditioned merger (which Hoenigswald calls "primary split"), in which some instances of phoneme A become an existing phoneme B; the number of phonemes does not change, only their distribution. Phonemic split (which Hoenigswald calls "secondary split"), in which some instances of A become a new phoneme B; this is phonemic differentiation in which the number of phonemes increases. Unconditioned merger, in which all instances of phonemes A and B become A; this is phonemic reduction, in which the number of phonemes decreases. This classification does not consider mere changes in pronunciation, that is, phonetic change, even chain shifts, in which neither the number nor the distribution of phonemes is affected. Phonetic vs. phonological change Phonetic change can occur without any modification to the phoneme inventory or phonemic correspondences. This change is purely allophonic or subphonemic. This can entail one of two changes: either the phoneme turns into a new allophone—meaning the phonetic form changes—or the distribution of allophones of the phoneme changes. For the most part, phonetic changes are examples of allophonic differentiation or assimilation; i.e., sounds in specific environments acquire new phonetic features or perhaps lose phonetic features they originally had. For example, the devoicing of the vowels and in certain environments in Japanese, the nasalization of vowels before nasals (common but not universal), changes in point of articulation of stops and nasals under the influence of adjacent vowels. Phonetic change in this context refers to the lack of phonological restructuring, not a small degree of sound change. For example, chain shifts such as the Great Vowel Shift in which nearly