It was this latter method to which

It was this latter method to which Filippo resorted m the summer of
1420. Downstream from Florence he had a large area of the Amo's bank
levelled, an expanse roughly half a mile in every direction, and in the sand
he traced a full-scale plan of the dome.^ It is most likely that the templates
for each of the eight vertical ribs were made from this enormous
geometrical design. These models, cut from pine, were S/, feet in length
and roughly 2 feet wide, bheets of iron were used to stiffen them and
prevent their warping. They were fitted on to the outside wall of the inner
shell, allowing them to serve as guides for both shells, which were built
with identical inclinations. Moved progressively upwards as the dome rose,
they en^Ull^d lli^L diL LJ^lii iiuaajvl iibs vvuuld ulLimately converge at the
level of the fourth stone rinp. In order for these ribs to serve as guides for
the rest of the dome's vertical curvature, they were built first: that is, only
after several courses of the bricks for the ribs were laid did the masons
begin filling in the intermediate sections.