Arc-boutant (flying buttress)

Flying buttresses are, as far as I am concerned, better known in gothique (Gothic) than in roman (Romanesque) architecture.
But that may be due to my own ignorance.
By lack of better examples, in the picture two flying buttresses (one behind the other) of the Notre-Dame in Cluny.
Flying buttresses transfer the horizontal forces from the superstructure to a pile or culée (buttress) at the end of the arch.
See also pile and culée.

Notre-Dame - Cluny