The rib, also known as an ogive, is an architectural element that divides a vault or dome into equal parts. It was mostly used in medieval architecture, especially Romanesque and Gothic, although it appeared in Roman architecture as early as the 2nd century AD. It serves to lighten the thrusts. In fact, thanks to the ribs, it is possible to build thinner walls and lighten the building. The ribs are placed according to longitudinal arches, forming triangular spaces known as sails.