This paper discusses a theory of redundantly specified phonological features as variable phonetic enhancement features (Stevens et al. 1986). It reports on an articulatory study of the American English vowel backing contrast and redundantly specified rounding contrast, showing how the theory of enhancement applies to this case. Analyses show tongue backing to vary considerably, while the presence of (redundantly specified) rounding acts to compensate for cases of reduced tongue backing. Lip rounding thus accounts for a consistent lowering in F2. The redundant specification of rounding makes the backness/rounding distinction more robust to co articulatory influences. These results are discussed with respect to various theories of phonological features and feature geometry.

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