Balancing the Figure's Weight at Its Center of Gravity
Stacking head, trunk and hips over the feet, and counterbalancing a carried load, with references to Libro A
A text page from the Trattato della pittura, Book Three, treating the mechanical balance of the painted figure. Leonardo first states that a figure's head must sit well over the shoulders, the trunk over the hips, and hips and shoulders over the feet. He then gives, with citations to his Libro A (folios 51 and 32), the principle that a still figure distributes equal opposite weights about the center of its support, so an arm thrown forward is answered by weight cast back. A further rule notes that a man lifting or carrying a load must project more than an equal counterweight to the opposite side.
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Stacking the body over the feet
Among the chief things required in depicting living creatures is to place the head well over the shoulders, the trunk over the hips, and the hips and shoulders over the feet. Correct stacking of these masses gives the figure its stability.
Equal opposite weights about the center of gravity
A figure that supports itself motionless on its feet gives off equal opposite weights around the center of its support. If it throws an arm forward before the chest, it casts as much natural weight backward as it throws of natural and accidental weight forward, and the same holds for any part that projects out beyond the usual (Libro A, folio 51).
Counterbalancing a lifted load
A man will never lift or carry a weight without sending out from himself more than an equal weight to the one he wishes to lift, projecting that counterweight to the side opposite the load. The rule governs how a burdened figure leans (Libro A, folio 51).