When viewing an infant from the side, an imaginary line may be drawn parallel to the face at the level of the maxilla (dotted line). In an infant with a normal airway, the mandible will approach this line. A mandible that is recessed well below this line, as shown by the arrow here, is a sign of micrognathia and a potentially difficult airway.
Reproduced with permission from Narasimhan Jagannathan, MD.