Right next to Notre-Dame you can visit the Musée de Notre-Dame, where paintings and prints show you what the building and neighborhood were like before the nineteenth century demolitions and restorations. Don't miss the depictions of how the church is set up and decorated for royal or religious festivals, either. You'll also be able to admire some Paris-related Christian objects, including an early Christian glass cup with the uppercase Greek letters "Chi" and "Ro", the two initials of Christ's name that make this perhaps the world's most ancient symbol.
