Extension of the line began on 29 December 1922, intending to transform Line 8 into a parabolic axis connecting Auteuil to Porte de Charenton via Opéra, Place de la République, Place de la Bastille, Avenue Daumesnil in the 12th arrondissement and Porte de Picpus. As part of the extension, a common route with Line 9 between the Richelieu – Drouot and République stations was planned in order to limit the impact of this problematic section on street traffic. Located on the former course of the Seine, the ground is soft and the initial plan to build two parallel tunnels generated lengthy controversy.