Autosegments were introduced by John Goldsmith in his 1976 MIT dissertation to represent tone and other suprasegmental phenomena. Goldsmith’s intuition, embodied in the term he created, was that autosegments constituted an independent, conceptually equal tier of phonological representation, with both tiers realized simultaneously like the separate voices in a musical score.
The analysis of suprasegmentals came late to generative phonology, even though it had been tackled in American structuralism with the long components of Harris 1944 and despite being a particular focus of Firthian prosodic analysis. The standard version of generative phonology of the era (Chomsky & Halle’s The Sound Pattern of English) made no special provision for phenomena that had been labeled suprasegmental or prosodic by earlier traditions.
An early sign that tones required a separate tier of representation was the phenomenon of tonal stability. In many tone languages, when vowels are lost historically or synchronically, their tones remain. The behavior of contour tones in many languages also falls into place when the contours are broken down into sequences of level tones on an independent level or representation. The autosegmental framework captured this naturally, since a sequence of elements on one tier can be connected to a single element on another. But the single most compelling aspect of the early autosegmental model was a natural account of tone spreading, a very common process that was only awkwardly captured by rules of whatever sort. Goldsmith’s autosegmental solution was the well-formedness condition, requiring, among other things, that every tone on the tonal tier be associated with some segment on the segmental tier, and vice-versa. Tones thus spread more or less automatically to segments lacking them. The condition of well-formedness, at the very core of the autosegmental framework, was a rare constraint, posited nearly two decades before optimality theory.
One-to-many associations and spreading onto adjacent elements are characteristic of tone but not confined to it. Similar behaviors are widespread in long-distance phenomena including intonation, vowel harmony, and nasal prosodies, as well as more locally with partial or full assimilation across adjacent segments. A major discovery, in Mark Liberman’s 1975 MIT dissertation, was that autosegmental tiers have hierarchical structure, with Goldsmith’s autosegments as the terminal elements of those structures.
The early autosegmental notion of tiers of representation that were distinct but conceptually equal soon gave way to a model with one basic tier—called the skeleton or CV tier—connected to tiers for particular kinds of articulation, including tone and intonation, nasality, vowel features, and others. This has led to hierarchical representations of phonological features in current models of feature geometry, replacing the unordered distinctive feature matrices of early generative phonology.
Autosegmental representations and processes also provide a means of representing nonconcatenative morphology, notably the complex interweaving of roots and patterns in Semitic languages.