Analyses of late gothic architecture contradict the common principles that the form was not pre-defined. João de Castilho’s vaults from the Manueline period clearly reveals the conception of the form based on a previous definition of the plan and elevation. This Spanish architect (1470 - 1552) is the creator of the best examples of ribbed vaults in all Europe. He introduces curved ribs in Portugal, aligned with the plane formed by the diagonal and tiercerons or outlining lowered plans at the top of the vault. In his last works, Castilho mostly built depressed barrel vaults, with depressed arches. We point out the importance of the monastery church of Jerónimos in Lisbon, where the space unit reaches perfection compared to other examples. The three naves are covered by a "single barrel vault", consisting of a complex network of ribs forming triangles which buttress one another instead of the traditional tranverse and longitudinal arches. The geometry is based on the principle of cross-ribbed vaults. The octagonal decorated pillars, are extremely slender with a 1m diameter. The vault of the transept reaches unique dimensions, 19m x 28.5 m without intermediate pillars, formed by two thick transverse arches and ribs in the shape of stars and circles. The plans of the nave vault found in archives reveal complexity in the construction, since the ribs are defined by arches of circumferences of different rays, not allowing for the standardization in the cutting of the ribs and in the manufacturing of the centering frames. In our recent research we verified the standardization of the ribs in almost 40 vaults surveyed, and the relationship between the design and the construction methods, particularly the evolution process: first the design/construction of the diagonal, than the location of the central key, placement of the rampante (section of the vault), location of the keys in the rampante and finally the design/construction of the tiercerons. A « step by step » method of design and construction of the ribs is applied. The analyses of several vaults surveyed led us to understand the principles used for the creation of big span vaults and introduces new ideas on the methods of planning and construction of ribbed vaults, especially for the creation of the form.