In the infant (Fig 1a), the narrowest point of the airway is at the level of the cricoid ring with the result that the airway has an inverted cone shape. Thus, the endotracheal tube diameter needs to be matched to the internal diameter of the cricoid ring.

In the adult (Fig 1b), the narrowest part of the airway is at the vocal cords and the airway has a cylindrical, parallel-sided shape. The changeover between these profiles is around puberty.