Leonardo wanted to create an ideal of human proportions. The idea came from Plato and Vitruvius, who pointed out that the proportions of the human body were analogous to the parts of an ideal temple.

On this sheet, Leonardo notes the proportions of a man’s head. The drawing is an external counterpart to his Studies of the Human Skull. Comparison between the drawing and accompanying notes indicates that the drawn proportions have been calculated by eye and are not precisely constructed. Leonardo writes that “the join of the lower lip with the chin and tip of the jaw and the upper tip of the ear with the temple forms a perfect square”, but the squares drawn on the face are uneven.

The distinctive model - a mature, bald man with sharp facial features and a square jawline, suits the format of the linear perspective “box” applied to his features. He appears in other proportional drawings by Leonardo. He also seems to have provided inspiration for the nude horseman drawn in red chalk below sometime later, probably in preparation for the Battle of Anghiari fresco.

In Leonardo's words
...each side by itself is half the head. The hollow of the cheek-bone occurs half-way between the tip of the nose and the top of the jaw-bone, which is the lower angle of the setting on of the ear, in the frame here represented. From the angle of the eye socket to the ear is as far as the length of the ear, or the third of the face.