Clients frequently ask why it is that they have difficulty singing on AH' /a:/. This may be a general problem, or something that happens in a certain part of the range or in the production of a particular voice quality. From the front, AH' looks as though it should be a good vowel to sing: the jaw and lips are relaxed with the tongue low and 'out of the way'. Acoustically 'EE' /i:/ is a more favourable vowel for singing. 'EE' is made by bringing the tongue forward and away from the back wall of the vocal tract, resulting in more space. By manipulating the tongue, the lips and the jaw, you can tune the oral cavity to enhance specific resonances. This chapter explores these structures and the effect they can have on resonating quality.