At the bottom of the nave, under the rose window, is hung the great organ of Notre-Dame de Paris. It is a huge musical instrument containing more than 8,000 pipes that the organist uses five keyboards (as if we had five piano keyboards one above the other!) and a pedalboard to make the organist speak. Each pipe works like a flute in which air is sent to produce a sound; but there are more than 8,000 of them, the tallest being more than 10 metres high and the smallest much less than 1 centimetre! It therefore requires large quantities of air stored in large bellows to make the instrument sing.
The organ is an important character in the cathedral, it accompanies the prayer of the faithful by supporting the singing or by intervening at particular times of the mass such as the entrance procession or the exit. It can be heard every Sunday afternoon during auditions given by organists from all over the world to play it.