In a modern church an aisle is thought of as a row down the middle of the church with a set of pews on each side. In a cathedral-type church building, an aisle is a passageway usually on both sides of the nave and separated from the nave by a row of pillars or columns. In a medieval cathedral a bride walking down either of the two aisles would hardly have been seen by people standing in the nave, which had no pews or chairs.
The earliest example is that found in the basilica of Trajan, which had double aisles on either side of the central area; the same number existed in the original church of St Peter's at Rome, in the basilica at Bethlehem, and according to Eusebius in the church of the Holy Sepulchre at Jerusalem.
The aisles are divided from the nave or central area by colonnades or arcades, and may flank also the transept or choir, being distinguished as nave-aisles, transept-aisles or choir-aisles.
In some of the churches in Germany the aisles are of the same height as the nave.