COSTOLONE. — Thus denominated is the arch placed generally diagonally which constitutes the load-bearing framework of vaults. The Romans were not unaware of this principle, though they applied it only for reinforcement and without projecting the structures from the surface. The projecting ribs of the Hispano-Moorish domes of the 9th–10th centuries served merely a decorative function; attempts at load-bearing arcades are seen instead in Armenian churches at the end of the 10th and beginning of the 11th centuries, but it is to Lombard builders that credit must be given for introducing this system and spreading II.
By the end of the 11th century it was adopted in S. Ambrogio and later in S. Nazaro Maggiore, both in Milan. In Latium
COSTOLONE — Vault with ribs of the church of S. Barbara, work by B. Rejt (1540–48) — Kutná Hora.
its companion appears around 1050 in S. Flaviano of Montefiascone and in the churches of Tarquinia; and in this same century they are also found in France and England (cathedrals of Durham and Peterborough).
In France transverse ribs across the nave were also used, frequent particularly in Normandy and Burgundy, and in the late Gothic period star vaults appeared, that is, with ribs intersecting in various directions, also in England, Germany, and Spain. In late Gothic the ribs are often mere decorative elements.
In Italy, on the other hand, ribs retained their original arrangement as diagonal arches even into the full 15th century (S. Agostino in Rome, S. Maria delle Grazie in Milan), as well as in Sicily and Sardinia where the influence of Aragonese Gothic is manifest.
In apses the ribs almost always have merely decorative value; in domes, however, the static function of ribs assumed decisive importance, forming the load-bearing system of the entire architectural organism (prime examples in S. Maria del Fiore, where the ribs made it possible to eliminate the centering, and in the Vatican Basilica).
In the 17th century Guarini also adopted intersecting ribs in domes, achieving daring structural feats (S. Lorenzo in Turin) surpassed only in the last century by Antonelli in S. Gaudenzio in Novara.
The section of ribs, initially rectangular, later became more complex with moldings until assuming a decorative character as well. Stone was mostly used as the material, but in Emilia brick and terracotta were also employed.