Charles arrives in Paris to stay with his cousin Paul; at the Gare de Lyon he asks a taxi-driver to take him to Neuilly -sur-Seine. The driver tells him he should just say Neuilly, 'it's shorter'. Technically Charles was right to be specific, since there is also a Neuilly-sur-Marne, and also a Neuilly-Plaisance, but for all Parisians the only Neuilly is Neuilly-sur-Seine, the wealthy suburb bordering Paris to the west, just above the Bois de Boulogne.
The actual address Charles gives, '11 rue des Sycamores', is a fake address; there is no such street in Neuilly. But the taxi does take him to Neuilly, depositing him outside number 11 rue des Dames Augustines:
This was Claude Chabrol's address at the time.