Abstract For a century, phoneticians have noted a vowel merger in middle-class Scottish English, in the neutralisation of prerhotic checked vowels /ɪ/, /ʌ/, /ɛ/ to a central vowel, e.g. fir, fur, fern [fəɹ], [fəɹ] [fəɹn], or [fɚ], [fɚ], [fɚn]. Working-class speakers often neutralise two of these checked vowels to a low back [ʌ] vowel, fir, fur, both pronounced as [fʌɹ] or as [fʌʕ]. The middle-class merger is often assumed to be an adaptation towards the UK’s socially prestigious R.P. phonological system in which there is a long-standing three-way non-rhotic merger, to [ɜː]. However, we suggest a system-internal cause, that coarticulation with the postvocalic /r/ may play a role in the contemporary Scottish vowel merger. Indeed, strongly rhotic middle-class Scottish speakers have recently been found to produce postvocalic approximant /r/ using a markedly different tongue configuration from working-class Scottish speakers, who also tend to derhoticise /r/. We present the results of an ultrasound tongue imaging investigation into the differing coarticulatory effects of bunched and tongue-front raised /r/ variants on preceding vowels. We compare tongue shapes from two static points during rhotic syllable rimes. Phonetically, it appears that the bunched /r/ used by middle-class speakers exerts a stronger global coarticulatory force over preceding vowel tongue configurations than tongue-front raised /r/ does. This also results in a monophthongal rhotic target for what historically had been three distinct checked vowels. Phonologically, our view is that middle-class speakers of Scottish English have reduced the V+/r/ sequence to one segment; either a rhoticised vowel /ɚ/ or a syllabic rhotic /r/.