As mentioned, the use of supporting buttresses to lighten the weight on the nave created a space between the nave and the support walls which 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 give insight into important French saints.
Saint Denis for example was the first bishop of Paris.
The legend goes that he was beheaded by the Romans on the hill of Montmartre, picked up his head and walked 10km, while giving a sermon, 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 archbishop Affre, shot while trying to engage with revolutionaries during the Revolution of 1848.
Saint Georges is known as the patron-saint of chivalry and Great Britain.
The story goes that he was a 4th century Greek officer who freed a village being terrorised by a dragon and was beheaded because he refused to persecute Christians.
His chapel at Notre Dame also contains the tomb of archbishop Darboy, shot during a popular uprising in 1871.
Saint Guillaume can refer to either a 12th century Italian noble who became a hermit performing miracles or a baker from Scotland, who was murdered by his adoptive son and became the patron-saint of adopted children.
The chapel contains the tombs of Jean Jouvenel des Ursins, the son of a cloth merchant who became president of the Parliament of Paris, and Henri d’Harcourt, Louis XIV's ambassador to Spain during the crisis of the Spanish succession.