The Diocletian window

In a bonded masonry wall the courses above an opening corbel over it of their own accord, so the lintel carries roughly the triangle of wall with sides rising at 60° from its ends; everything above that triangle arches onto the jambs.

load ≈ equilateral triangle on the span: W ≈ 0.43 · L² · t · ρ g

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

Lift the dome on a cylindrical or polygonal drum pierced with windows between the ring of the pendentives and the springing of the dome, to light the centre of the church and to make the dome visible from afar.

drum height ≈ ⅓–1 × its radius (Byzantine); much taller in Russian churches

Make the architrave round a door or window as wide as a sixth part of the opening’s breadth, and dress the openings of the principal floor more richly than those of the storeys above and below, with a frieze and cornice, and on the grandest a pediment.

architrave = ⅙ × width of the light

Over a long row of windows on the principal floor, set triangular and segmental pediments in turn, so that the row is read in pairs and does not run on monotonously.