Dürer's work on human proportions is called the Four Books on Human Proportion (Vier Bücher von menschlicher Proportion) of 1528. The first book was mainly composed by 1512/13 and completed by 1523, showing five differently constructed types of both male and female figures, all parts of the body expressed in fractions of the total height. Dürer based these constructions on both Vitruvius and empirical observations of "two to three hundred living persons", in his own words. The second book includes eight further types, broken down not into fractions but an Albertian system, which Dürer probably learned from Francesco di Giorgio's De harmonica mundi totius of 1525. In the third book, Dürer gives principles by which the proportions of the figures can be modified, including the mathematical simulation of convex and concave mirrors; here Dürer also deals with human physiognomy. The fourth book is devoted to the theory of movement.
