In General American English (GAE), only two full vowels [a, C] occur in syllables ending in [r] plus a non-coronal consonant, e.g. <harp>, <pork>. An articulator study of rhotic production by tree speakers of GAE was conducted using real-time structural Magnetic Resonance Imaging (rtMRI) (Narayanan et al. 2004). Subjects produced /r/ in simple and complex syllable codas in a range of vocalic environments. Results show that th e tongue dorsum shows the least movement in [-ar-] and [-Cr-] sequences. This dorsal stability sheds light on why [a] and [C] are the only full vowels occurring before codas with /r/ and a non-coronal consonant. English syllable rimes have been analyzed as maximally tree timing units in length (Hammond 1999). Long vowels occupy two units, and most coda consonants occupy one unit, rendering rimes with long [a]/[C] followed by [rp] or [rk] problematic. We hypotesize that th e high degree of overlap in the dorsal posture in [ar] and [Cr] sequences allows their gestures to be partially blended and function like a diphthong tat occupies two units in the rime. This study supports a view of maximal constituency in rimes with [r] tat takes articulator overlap into account.