Navigators are a form of frame that is actually an evolution of aviators, as it also comes from the needs of American military pilots. The transition to jet aircraft entailed a change in equipment: different helmets and headsets, and most importantly, the introduction of an oxygen mask that covers part of the face. This led to improvements: a reduced rectangular shape with, so to speak, a flat bottom of the eyepiece, as opposed to the convex teardrop shape of classic aviators.