- Internal compartments of a building; each divided from the other by subtle means such as the boundaries implied by divisions marked in the side walls (columns, pilasters, etc.) or the ceiling (beams, etc.). Also external divisions of a building by fenestration (windows).
- A section of a building distinguished by vertical elements such as columns or pillars. Often, a bay will protrude from the surface of the wall in which it is situated, thus creating a small, nook-like interior space, often of a rectangular or semi-hexagonal outline. See bay window.