Leonardo da Vinci's drawing is so familiar it feels like a diagram, and in a way it is. Around 1490 he set out to illustrate a passage in the Roman architect Vitruvius, who claimed that the human body fits perfectly into both a circle and a square. Many artists had tried to draw it. Leonardo solved a problem that others had fudged.
The trick is that the circle and the square do not share a center. In Leonardo's drawing, the circle is centered on the navel, and the square on the groin. The man stands with his feet together inside the square, and with arms and legs spread he touches the circle. As he moves from one pose to the other, the center of his body seems to shift. It is a small piece of geometry hidden in plain view.
The notes are part of the work. Leonardo wrote in his usual mirror writing, right to left, and the text above and below the figure lists the proportions that Vitruvius described: that the span of the arms equals the height, that the palm is four fingers wide, that the foot is a sixth of the height. Where he found the ancient text wrong, he corrected it from his own measurements.
The sheet is small and fragile, drawn in pen and ink on paper, and light damages it. It belongs to the Gallerie dell'Accademia in Venice and is shown only on rare occasions.
Its image is everywhere. It is on Italy's one euro coin and in countless medical and science logos, a figure that stands for the idea that human proportion can be understood, and that art and science are one project.
Leonardo's Vitruvian Man appears on our: