The cut or depression below the lower lip of the mouth is half way between the bottom of the nose and the bottom of the chin.
The face forms a square in itself; that is its width is from the outer corner of one eye to the other, and its height is from the very top of the nose to the bottom of the lower lip of the mouth; then what remains above and below this square amounts to the height of such another square, a b is equal to the space between c d; d n in the same way to n c, and likewise s r, q p, h k are equal to each other.
It is as far between m and s as from the bottom of the nose to the chin. The ear is exactly as long as the nose. It is as far from x to j as from the nose to the chin. The parting of the mouth seen in profile slopes to the angle of the jaw. The ear should be as high as from the bottom of the nose to the top of the eye-lid. The space between the eyes is equal to the width of an eye. The ear is over the middle of the neck, when seen in profile. The distance from 4 to 5 is equal to that from s to r.
[Footnote: See Pl. VIII, No. I, where the text of lines 3-13 is also given in facsimile.]
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.