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|Sound change and alternation|
In historical linguistics, a chain shift is a set of sound changes in which the change in pronunciation of one speech sound (typically, a phoneme) is linked to, and presumably causes, a change in pronunciation of other sounds. The sounds involved in a chain shift can be ordered into a "chain" in such a way that after the change is complete, each phoneme ends up sounding like what the phoneme before it in the chain sounded like before the change.[specify] The changes making up a chain shift, interpreted as rules of phonology, are in what is termed counterfeeding order.[clarification needed]
A drag chain or pull chain is a chain shift in which the phoneme at the "leading" edge of the chain changes first. In the example above, the chain shift would be a pull chain if /i:/ changed to /aɪ/ first, opening up a space at the position of [i], which /e:/ then moved to fill. A push chain is a chain shift in which the phoneme at the "end" of the chain moves first: in this example, if /aː/ moved toward [eː], a "crowding" effect would be created and /e:/ would thus move toward [i], and so forth. It is not known which phonemes changed first during the Great Vowel Shift; many scholars believe the high vowels such as /i:/ started the shift, but some suggest that the low vowels, such as /aː/, may have shifted first.