Luca Pacioli, known as Brother Luca, was a Franciscan monk and a mathematician. In 1497, he was invited to the court of Lodovico Sforza in Milan, where Leonardo de Vinci (1452-1519) was also in residence. The two became friends and spoke at length about mathematical theory as it applied to the application of proportion in artistic composition. It was at this time that Luca began his book De divina proportione. Leonardo provided some of the illustrations and the book was dedicated to Lodovico. In 1499, Luca and Leonardo were forced out of Milan and it was not until 1509 that their three volume work was published in Venice (volume three is an Italian translation of Piero della Francesca’s Latin writings On [the] Five Regular Solids). For more information, read Ruth Mortimer’s Harvard College Library Department of Printing and Graphic Arts, catalogue of books and manuscripts, (1964, v.2, part 2, p. 499-502. GA Z881 .H346)
Divine Proportion, or the golden ratio, is the ratio a : b = b : (a + b).