The arc rampant, as it is known in French, is an arch in which the starting points of the arch are not the same height. They seem to be found in side aisles to support the arcade walls.
Sometimes these arc rampants seem to be accidental, or later additions, so it took awhile before I discovered that they were intentional structures. They usually function as internal buttresses, similar to flying buttresses that appeared later in Gothic architecture.
This is a rare feature which I can think of in only a few churches off the top of my head; the Cathédrale Notre Dame de Nazareth in Vaison-la-Romaine, the Église Saint-André-de-Sorède, and the south side aisle of the Abbaye Saint Junien in Nouaillé-Maupertuis near Poitiers.
This is part of a series of posts featuring an amuse-bouche, a bite-sized appetizer to whet the appetite of diners. Each of these will explore a single interesting feature of medieval architecture or sculpture. To see other amuse-bouches, follow this link.