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What are the factors that shape linguistic sound systems? Perceptibility of the acoustic signal has long been argued to play a role in phonological organization.  Some theories of historical change (such as Ohala 1993, 2012; Blevins 2004) argue that sound change results from misperception of the acoustic signal, while teleological models of phonology (Lindblom 1990; Hayes et al. 2004) posit that speech is optimized  (in part) for auditory perceptibility. Nevertheless, it is well known that speech perception is influenced by a range of non-auditory cues (McGurk & MacDonald 1976; Gick & Derrick 2009; Mayer et al. 2013). Here I discuss the role of visual cues in language  variation and change. A cross-linguistic survey of sound changes involving labial segments, including development of the rare labio-coronals in Setswana, suggests that segments tend to retain their labiality in order to maximize perceptibility in both the  auditory and visual domains. That hypothesis is further tested in an articulatory and perceptual investigation of the Northern Cities Vowel Shift. Using ultrasound tongue imaging, I find that speakers exhibiting the NCVS vary in how they produce the contrast  between the vowels /ɑ/ and /ɔ/. An audiovisual perception experiment demonstrates that variants of fronted /ɔ/ that retain their lip rounding are less likely to be misperceived as /ɑ/ than variants that are produced with unround lips. Together, these results  show that consideration of visual speech perception can improve our understanding of cross-linguistic patterns of variation and change. Theories of sound change and of phonological typology must therefore take into account visual as well as auditory perceptibility.