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Paris Métro Line 12 (French: Ligne 12 du métro de Paris : initially opened as Ligne A du Nord-Sud (North-South Line A)) is one of the sixteen lines currently open on the Paris Métro. It links Issy-les-Moulineaux, a suburban town southwest of Paris, to Aubervilliers, in the northeast. With around 54 million passengers per year, Line 12 was the twelfth busiest line of the network in 2021. It services several major stops, such as Porte de Versailles, Concorde, Madeleine, and two national railway stations, Gare Montparnasse and Gare Saint-Lazare. The service runs every day of the week, and the line uses MF 67 series trains, the network's standard since the early 1970s.
Line 12 was founded in 1905 as Line A by the Nord-Sud Company, who also built Line 13. It was built between 1905 and 1910, to connect the districts of Montparnasse, in the south, and Montmartre, in the north of the French capital. The first trip, from Porte de Versailles to Notre-Dame-de-Lorette, was on 5 November 1910. The line was the second to be built on the north–south axis of the city, in competition with Line 4 of the Compagnie du chemin de fer métropolitain de Paris (CMP; Paris Metropolitan Railway Company). It was extended bit by bit until 1934 when it reached Mairie d'Issy in the south. Tunnelling to the northern terminus at the Porte de la Chapelle on the perimeter of Paris had been completed in 1916. In 1930, the CMP bought the failing Nord-Sud company and Line A was integrated into the new, unified network as Line 12. In 1949, the CMP was itself merged into the RATP, Paris's public transport company, which still operates the line today.