Da Vinci's sketch of the Vitruvian Man shows the proportions of the human body: that each separate part was a simple fraction of the whole. For instance, the head measured from the forehead to the chin was exactly one tenth the total height. The outstretched arms were always as wide as the body was tall. (It's a generally applicable rule that your arm-span matches your height—go ahead and test it out).
Vitruvian Man was named for Vitruvius, a mathematician that practiced mathematical ideas for Pythagoras. In this drawing, da Vinci tests Vitruvius's theory that a man's proportions would fit evenly in both a square and a circle (the spiritual realm and physical realm).