By 1418 the octagonal drum of Florence Cathedral stood with an open problem: spanning its 45-metre opening seemed impossible with the techniques of the day. The vast timber centring a dome would normally require was neither economical nor practical. The solution came not from a trained architect but from Filippo Brunelleschi, who had trained as a goldsmith.

Brunelleschi used two linked innovations. The first is the double shell: two separate domes, an inner one thick and load-bearing, an outer one thin and protective, with a gap between them through which stairs run. This both reduces weight and separates the outer surface against rain. The second, and more radical, is laying the bricks in a herringbone (spina-pesce) pattern: bricks set upright between the horizontal courses lock the unfinished courses so that each ring carries itself. The dome could thus be built ring by ring, rising without any supporting centring from below.

The dome was also girdled with embedded stone and timber tension chains to resist the horizontal thrust — like the hoops of a barrel. Brunelleschi designed special hoists and lifting machines for the construction too. The Florence Dome is therefore not only a form but a method of engineering; it inaugurates the Renaissance figure of the architect as also a problem-solving technician.