A series of historical sound changes. Two kinds of chain shift have been hypothesised:
- Drag-chain: a change in quality over time leaves a phonetic gap into which another unit moves, which in turn leaves a phonetic gap and so on. For example, the English Great Vowel Shift could be explained by positing the first event in time to be the diphthongisation of the high vowels. This left a gap in the vowel system into which the high-mid vowels moved and so on.
- Push-chain: a change in quality over time means that one phonological unit becomes too close to another. The second unit begins to move until it too becomes too close to a third and so on. Under this hypothesis, the English Great Vowel Shift began with a raising of the low vowels, which exerted pressure on the mid-vowels and so on. Finally, the high vowels became diphthongs.