In his classic and still widely-used textbook on drawing the human form, Master Class in Figure Drawing, Robert Beverly Hale examines the various components of the body as drawn by exemplary Old Masters and discusses their complexity for the draftsman. Hale has this to say about the eye:
Artists sometimes drop a center line from the supraorbital bump. The growing back of the down plane of the glabella is clearly in directional contract to the up plane of the nasal bone. The orbit of the eye moves back, down, and around. The essential thing is to feel the ball of the eye protruding from the opening of the orbit and the lids circling over that ball. But that's not quite the whole story because . . .My point is that, as Hale knew, the eye is a complex visual study requiring a knowledge of anatomy—and even of physiology—on the part of the artist.
Windows, as the eyes of a building, should be rendered in equally complex terms and should demand of the architect and the builder an attention every bit as great as that which Michelangelo or Rembrandt paid to rendering the human eye.
Unless they are very precocious, children draw eyes (and faces in general) in two dimensions, not three:
This is the equivalent of architects, designers, and builders who settle for two-dimensional fenestration, usually off the shelf from The Home Depot.
Like eyes, windows are three-dimensional features, not two-dimensional openings. Real windows have depth as well as height and width.