Building a cathedral was a gargantuan task.
One of the most complex parts was building the nave.
Fun fact : the word "nave" means "ship" in Latin - the church saw itself as a ship, guiding the congregation to salvation.
One of the main complexities of the nave stems from its arched ceiling.
While architects knew how to build arched ceilings since Antiquity - Egyptians and Romans had massive arched ceilings - these were very weak.
The top of the arched ceiling would push downwards and the supporting walls push outwards - so walls needed to be extra thick to withstand the pressure.
Arched ceilings meant buildings were extremely low and dark as any window would have weakened the strength of the walls.
In the 11th century, architects discovered a new technique, the rib vault, created by the intersection of 2-3 arches.
With this system, pressure was diverted to 4 supporting pillars - not the walls themselves.