In a row of arches the thrusts of neighbouring arches cancel over each intermediate pier, which may therefore be slender; at the ends of the row nothing balances them, so the outermost piers must be made wider or buttressed.
Divide the wall into bays
Run a shaft or pilaster from the floor, up the face of the pier and the wall above, to the roof or vault, so that the long nave wall is divided into a sequence of vertical bays, each containing one arch of the arcade and the windows above it.
Early Christian basilicas have continuous walls over their colonnades. Eleventh-century Normandy and the Rhineland introduced the bay: the wall organised by vertical members into repeated units. With vaults, the vertical shafts became the supports of transverse arches; even under timber roofs they gave the nave a measured, processional rhythm. The bay is the unit on which all later medieval design is built.
Related rules
Over a doorway or window spanned by a lintel, turn an arch in the wall above so that the weight of the masonry is carried round the opening onto the jambs, leaving the lintel to carry only the small triangle of wall beneath the arch.
Raise the central nave above the lean-to roofs of the aisles and pierce the upper walls with a row of windows, the clerestory, so that the middle of a wide hall is lit from high on both sides.
nave height ≈ 1½–2 × nave width; one window over each bay of the arcade
Gather the weight and thrust of the vaults onto a few points by ribs, carry them down piers and out across the aisles by flying buttresses to external buttress piers, and the wall between the supports carries almost nothing, so it can be opened into windows.
Divide the elevation of the nave into three storeys: a tall arcade opening into the aisles, a low triforium passage in the thickness of the wall against the aisle roof, and a clerestory of large windows reaching up into the vault.
Chartres: arcade ≈ clerestory, triforium a narrow band between
Proportion the cross-section of a church from the square: let the total height to the crown of the nave vault equal the total width, and take the heights of the aisles and the springing of the vaults at whole divisions of that square.
H = W · levels at k × (W ⁄ n)
Cite this page Chris Paton, “Divide the wall into bays”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/rom-bay-articulation. Text, drawings and models licensed under CC BY-NC 4.0.