On page 14 of the travel section is a photo of the tenor apparently singing
a high note on "a" or "o." This is an excellent example of the purely
vertical mouth opening I mentioned in a previous post. The tenor's lower
jaw is dropped to the maxium, and there is no hint of trying to widen the
sides of the mouth. This mouth position forms, in effect, the bell or
mouth of a trumpet and causes the tone to go up into the nasal passages.
If this mouth position can be assumed while keeping the tongue, throat, and
jaw relaxed (without any tension that doesn't help voice production), it
works very well and should produce a good, resonant, ringing tone.