During his final years Dürer endeavoured to support practice with theory. He had already prepared the first two volumes of his treatise on proportions before his journey to the Netherlands, and he continued working on them after his return. Due to the numerous geometrical drawings and illustrations needed to serve his didactic purposes, and the necessary study of classical sources, the completion of the work on proportions was delayed. Since his first stay in Venice, Dürer had worked on the theory of the ideal human proportions. Dürer linked the depiction of body types with differing proportions to the teaching of the four humours, and as such was the first to indicate the connection between build and character. The first volume dealt with basic geometrical terms such as the point and line, the second dealt with surfaces and bodies, and the third and fourth volumes were dedicated to both the ideal depiction of bodies and the construction of columns, letters and sundials. In the appendix, the reader, artist and craftsman can find a short treatise on perspective and instructions for the use of drawing equipment. The treatise on proportions was published posthumously by Willibald Pirckheimer and Dürer's wife Agnes.The picture shows an illustration from the Four Books on Human Proportions, Book I, published posthumously by Willibald Pirckheimer and Dürer's wife Agnes.The insights regarding measure and numbers which Dürer had gained from his lifelong study of the ideal proportions of the human body was summarized in his Four Books on Human Proportions. The head was used as the comparative yardstick for measuring the other parts of the body.