I have not seen it advanced elsewhere, but personally I have noticed this peculiarity in almost all faces- and even slightly marked in the faces of children: that the eyes run down either on the right or the left and that the mouth runs up toward the lower eye.
You will rarely see the eyes and mouth sloping the same way. The converging of these features on one side seems somehow to restore the balance of the face.
In the full face mark the nature of the triangle made of the ends of the brows and the tip of the nose, the distance between the eyes, the relative size of the chin and forehead to the nose; and ascertain whether the face inclines to the long, the round or the square.
Further than this I can only advise you, when drawing a face, after having examined its characteristics in the hand-glass, to place your drawing- if it be of large size- by the side, or preferably when possible in a line under, the face, and to look at it with your eyes half closed. See that the masses- that is to say, the areas of light and shadow- tally with those of the sitter; and that the head is correctly placed on the shoulders, about which there need be no special difficulty, and no loss of the real character, if only the background spaces be your constant guide. To this it is necessary to give careful consideration, for the setting of the head on the shoulders is an important factor in the realisation of character.The rest must depend on your own powers of discernment and draughtsmanship.¹
¹ Solomon J. Solomon, The Practice of Oil Painting & Drawing, (Seeley, Service & Co. Ltd.: London, 1952), pp. 53-54.