Botticelli’s Lehman Annunciation, a widely admired but seldom studied painting, is constructed in a unique fashion. The painter transferred the fully developed composition from paper onto his panel by means of inscribing the details of the architectural structure with a stylus into the soft gesso. Though he often used this technique to set the general framing or placement of figures he never previously drew the entire composition in detail on the panel. This decision suggests that he was intent on the precise placement of each line in the architectural frame. The golden section, an irrational proportional relationship between two unequal dimensions, may have been partly responsible for Botticelli’s method. The dimensions of the composition as well as the placement of other elements of the architecture reflect the mathematical proportions of 1:1.618, the numerical statement of the ratio. The golden section had been known in the Quattrocento as an incidental part of Euclid’s Elements but was not separately published until Luca Pacioli’s De divina proportione of 1509. Prior to Botticelli’s Annunciation it had not appeared in paintings, efforts to prove the contrary notwithstanding. Its use here was apparently the result of a connection between Botticelli and Pacioli in Florence in 1492, a firm date for the previously undated painting. Pacioli, a Franciscan monk, believed that the golden section was an expression of the ineffable nature of God, thus his name ‘divine proportion’. Botticelli, in the sway of the charismatic mathematician, seized on the proportion as a way of expressing visually the miracle of the creation of the Son of God.