Final answer:
A rib that spans the longitudinal axis of rib vaults and divides them into bays is known as a transverse rib; this feature is common in Gothic cathedrals and aids in the vault's structural and aesthetic design.
Step-by-step explanation:
A rib spanning the longitudinal axis of a rib vault that divides it into bays or compartments is called a transverse rib. This component is instrumental in the architectural design of rib vaults, which are characteristic of Gothic cathedral ceilings and are also found in other historical buildings. Transverse ribs are a key element in dividing the vaulted space into structurally manageable sections called bays and aid in the distribution of weight down to the columns or walls below.
The intricate nature of rib vaulting progressed over time, eventually incorporating additional ribs such as lierne and tierceron ribs to create more complex web-like structures. These advancements allowed for artistic and structural innovation, leading to elaborate designs such as the Perpendicular or International Gothic style's fan vaulting, highlighting the evolution of rib vaulting over the centuries.