Vol. I: VII. On the Proportions and on the Movements of the Human Figure: 319.
The Da Vinci Notebooks
Proportions of the head seen in front (319-321).a n o f are equal to the mouth.
a c and a f are equal to the space between one eye and the other.
n m o f q r are equal to half the width of the eye lids, that is from the inner [lachrymatory] corner of the eye to its outer corner; and in like manner the division between the chin and the mouth; and in the same way the narrowest part of the nose between the eyes. And these spaces, each in itself, is the 19th part of the head, n o is equal to the length of the eye or of the space between the eyes.
m c is 1/3 of n m measuring from the outer corner of the eyelids to the letter c. b s will be equal to the width of the nostril.