A transverse rib (French : arc doubleau) is a term in architecture given to the rib structure of a rib vault which runs the nave, dividing it into bays. In Roman architecture Transverse ribs were often sunk in or below the barrel vault of thermae , as in the piscina at Baiae and the so-called Baths of Diana (Nymphaeum) at Nîmes. In the Romanesque and Gothic styles of church architecture, Transverse ribs are principal feature of the vault, so much so that Scott [1] termed it the "master rib". [2]