The spaces between posts, columns, or buttresses in the length of a building, the division in the widths being called aisles. This meaning also applies to overhead vaults (between ribs), in a building using a vaulted structural system. For example, the Gothic architecture period's Chartres Cathedral has a nave (main interior space) that is "seven bays long." Similarly in timber framing a bay is the space between posts in the transverse direction of the building and aisles run longitudinally.
The openings for windows in a wall. For example in Georgian style, at Mulberry Fields, the building is described as a "5 bay by 2 bay facade," meaning a "5 windows by 2 windows" exterior.