Leonardo tested the Roman architect Vitruvius's claim that a well-made body fits both a circle and a square. One nude man takes two poses at once: arms level inside the square, arms and legs spread inside the circle. Leonardo made the scheme work by centring the circle on the navel and the square on the genitals.
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Tiny tick marks along the arms and legs record Leonardo's measurements, made with compass and ruler. The notes above and below the figure are in his mirror writing.
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