Tracings from cine x ray of the vowels of four speakers of Twi clearly demonstrated that vowels from harmony set 1 [/i,e,o,u/] systematically differed from vowels of harmony set 2 [ι,ε,(a),ɔ,ο/]by different vocal tract shapes: set 1 vowels had a wider pharynx and an advanced tongue root, while set 2 vowels had a narrower pharynx and a retracted tongue root. In spite of their different vocal tract shapes the two vowels /ι/ and /e/ have very similar formant frequencies. The difference in the vocal tract shapes is that /e/ has a wider pharynx and also a wider mouth cavity than /ι/. The fact that these vocal tract shapes produce similar formant frequencies can be explained with reference to perturbation theory: the area functions of these two vowels are approximately symmetric about the midpoint. Similar acoustic and articulatory data were found for one speaker of Ijo.