Vowels are traditionally viewed as one of the two major classes of speech sound. Vowels lack contact between the tongue and the roof of the mouth, and they are normally voiced. Importantly, the speaker receives little proprioceptive feedback from their speech organs, meaning that it is not fully appropriate to define a vowel in terms of place and manner of articulation. Instead, tongue height and advancement are used to describe vowel quality, and auditory features play a more important role in description, e.g. with reference to the Cardinal Vowels system. Vowel categories form a continuum, and this has repercussions in terms of articulatory, auditory and acoustic overlap between contrastive categories and the special role that vowel gradience plays in dialect variation and language change. Other features of vowels stem from the general openness of vocalic articulations. For example, in principle, lip rounding can be combined with any tongue position, and nasality too demonstrates specific patterns that have both phonetic and phonological dimensions. Vowel quantity, or duration, is also used in differing ways by different languages, both phonemically and allophonically.

The approach to language variation developed over the last half century has focused on systematic variation of variants for a single structural unit under the construct of the linguistic variable. While this approach may offer important detail for particular variables, it is essential to examine the overall configuration of a language variety as well. Composite linguistic measures facilitate the exploration of social factors and offer a profile of change for the entire linguistic system. We describe the methodological tools to provide a global description of language variation, “the composite index”, and to fully capture when and why individuals change their linguistic behavior over time. We emphasize the need to view language as a system by moving beyond individual variables to more fully characterize the ways in which individual linguistic features move in-tandem across the lifespan. The study demonstrates that a composite index score like the Dialect Density Measure (DDM) can be used as a means of tracking trajectories of language use across the lifespan on a unidimensional scale.