What is "transverse rib"

A transverse rib is the term in architecture given to the rib of a rib vault which is carried across the nave, dividing the same into bays. Although as a rule it was sunk in the barrel vault of the thermae, it is found occasionally below it, as in the Piscina at Baiae and the so-called Baths of Diana ( Nymphaeum) at Nîmes. In the Romanesque and Gothic styles it becomes the principal feature of the vault, so much so that Scott termed it the "master rib".