The use of flying buttresses, to lighten the weight on the nave, created a space between the nave and the support walls.
This could be filled with chapels.
Notre Dame has 29 of them - many of which were dedicated to patrons of the cathedral and famous nobles.
They also give insight into important French saints.
Saint Denis for example was the first bishop of Paris.
He is said to have been beheaded by the Romans on the hill of Montmartre, then picked up his head, while giving a sermon, and walked 10km before dropping dead on the future location of the royal basilica of Saint-Denis.
He is now the patron-saint of Paris.
His chapel at Notre Dame also contains the tomb of the archbishop Affre, shot while trying to appease revolutionaries in 1848.
The chapel of Saint George, honours the patron-saint of chivalry and now the patron-saint of England.
The legend goes that he was a Greek officer in the 4th century who freed a village being terrorised by a dragon and was beheaded when he refused to persecute Christians.
His chapel at Notre Dame also contains the tomb of archbishop Darboy, shot during the uprising of the Commune in 1871.
The chapel of Saint Guillaume can refer to either a 12th century Italian noble, who became a hermit, or a baker from Scotland, who was murdered by his adoptive son and became the patron-saint of adopted children!
The chapel also contains the tomb of Jean Jouvenel des Ursins, the son of a humble cloth merchant of the 14th century, who rose to become the president of the Parliament of Paris.
It also contains the tomb of Henri d’Harcourt, Louis XIV's ambassador to Spain during the crisis of the Spanish succession.