The Bishop of Notre-Dame uses his Crozier to Knock on the Cathedrals Great West Doors during a set of Call and Responses in order that the Procession may enter on Palm Sunday
For those of you who perhaps are unaware, Notre-Dame is the Cathedral for Paris...Le Sacre Coeur whilst far more prominant in location is a Minor Basilica. Cathedrals are more important...infact Basilica do NOT have Bishops.
This is true even in the case of The Pope. Many assume he sits on his Cathedra in Saint Peters Basilica...which is false. The Seat of the Bishop of Rome is actually in Saint John Lateran Cathedral. This Cathedral is also called a Major Basilica...but first and foremost its a Cathedral. The other Major Basilicas, of which their are only a total of four, one obviously being Saint Peters, are still, technically, lesser then Cathedrals. Bet you didnt know that did you.