II. Description of the basilica. 1. The quadratum populi. If we keep to the theoretical model, the nucleus of the Christian church building, from the peace of the church, was a rectangular hall generally with three aisles, completed by an apse which was originally of the same width as the main aisle and had the main entrance opposite it. Sometimes the main hall is given the name quadratum populi. The largest churches had five aisles e.g., the great Roman basilicas and sometimes even more up to seven or nine for some phases of the great churches of Carthage and Tipasa in Africa. The number of aisles depended not just on the dimensions but also on the strength of the supports used. In N Africa, for example, there were many relatively small five-aisled churches e.g., that of Orlansville. Various small single-aisled churches are also known, esp. in eastern Europe. The nave was normally much wider than the aisles the ideal proportion is 2:1, but regional schools show many variations.
The supports, resting either on a continuous sill stylobate or on separate foundations surmounted by a flat stone, called an underbase, under the base, were ordinarily columns, often reused, esp. at Rome. But there are some regions where pillars, either constructed of cut stone or sometimes monolithic, were substituted for columns. Many variations are known: columns attached to pillars e.g., the type of Tebessa in N Africa, double columns very frequent in Tunisia, columns of stuccoed brick, columns made from stacked pieces etc. The column was surmounted by a classical capital, often reused. Specific types were subsequently created, adapted for arches on columns by the addition of an abacus or superabacus, then the monolithic impost-capital and then the pyramidal capital. Above the columns the architrave of classical architecture retained where numerous decorative trabeations were available, at Rome and Constantinople was normally replaced by the arch, in brick or stone, which allowed wider spacing of the supports and resisted vertical pressure better transmitting it to the supports.