Chain Shift

In phonology, a chain shift is a phenomenon in which several sounds move stepwise along a phonetic scale. 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. The rules making up a chain shift are said to be in counterfeeding order.

For example, if in some language the three vowel phonemes /æ ɛ e/ undergo a change such that /e/ becomes /i/, /ɛ/ becomes /e/, and /æ/ becomes /ɛ/, those three changes would constitute a chain shift and could be summarized as

æ → ɛ → e → i

A drag chain or pull chain is a chain shift in which the phoneme at the "leading" edge of the chain changes first. In this example, the chain shift would be a pull chain if /e/ changed to /i/ first, opening up a space at the position of which /ɛ/ 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 /æ/ moved toward, creating a "crowding" effect and causing /ɛ/ to move toward, and so forth.