I’m a bit dyslexic so for me the acronym for <<Régie Autonome des Transports Parisiens>> “ratp” gets rearranged badly “:-)”. However their services and website are so fabulous that I am never discouraged.
I just finished providing a detailed custom bus plan for a client who is staying at the Marriott in the 14th arrondissement and wanted to bus over to the Louvre. I was delighted when I found that bus line 21 is the fastest, quickest and most convenient RATP conveyance. A 5 minute walk to the bus stop with an 18 minute ride through much of the beautiful and interesting 6th arrondissement is simply the best way to get from this hotel to the Louvre. Bus line 21 (direction Gare Saint-Lazare) will take you by the Panthéon, Jardin du Luxembourg, and La Sorbonne, crossing la Seine over the Boulevard Saint Michel with a view of Notre Dame and right past Sainte-Chapelle. The bus then turns left after crossing la Seine, traveling down the Rue Du Rivioli. To get to the Louvre you decend at the Palais Royal-Musée du Louvre stop. Cross the street and Voilá.
If you return by starting at the Palais Royal-Musée du Louvre stop, you’ll pass by the Operá Garnier before retracing the route through the 6th. Or you can start your return trip at the Palais Royal–Comédie Française but my current Line 21 map indicates that particular stop is non UFR accessible, and the ride past the Opera house is very pleasant so <<pourquoi pas?>>.