End the choir in a semicircle of piers; wrap an aisle (the ambulatory) around it, and open a ring of chapels off the ambulatory, each with its own altar and windows, so that the east end steps down in tiers of roofs and buttresses.
From the atrium to the apse
Order the church as a single path: a colonnaded atrium, a narthex, the long nave drawn toward the altar, and the apse framed by a triumphal arch at the end of the axis.
The basilica is a building for processions and for a congregation facing one way. Its sequence of spaces moves from the city to the sanctuary by degrees: the open atrium with its fountain, the covered narthex where those not yet baptised might stand, the nave, and finally the raised apse with the bishop’s throne behind the altar. Many early basilicas faced the entrance to the east, as Old St Peter’s did; the orientation with the apse to the east became the rule later.
The longitudinal axis, set against the centralised plan of baptisteries and martyria, is one of the two great spatial ideas of Christian architecture. Byzantine builders combined them; the Latin West kept the long nave through the whole Middle Ages.
Related rules
Make a basilica between a third and a half as broad as it is long; make each aisle a third of the width of the central space, and the columns as high as the aisles are broad.
W = ⅓ to ½ L; aisle = ⅓ nave; column height = aisle width; upper columns ¼ smaller
Lay out the church as an Early Christian basilica (a tall nave lit from above, lower aisles on either side, a sanctuary ending in an apse to the east) and cross it with a transept, so that the plan becomes a Latin cross with a tower over the crossing.
Let the square of the crossing set the measure of the whole church: the nave and transept arms are multiples of it, each nave bay is a square of the same size, and each aisle bay a square one quarter its area.
nave bay = crossing square; aisle bay = ¼ of it; nave length = n squares
Multiply altars at the east end by opening apses off the transept arms and the choir, stepped back from the main apse in echelon, each with its own small semidome and roof.
Mark the west end with a massive, towered block containing an entrance hall below and a chapel or gallery above, rising higher than the nave: a church in front of the church.
Cite this page Chris Paton, “From the atrium to the apse”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/basilica-axis. Text, drawings and models licensed under CC BY-NC 4.0.