In architecture, the double bay system (or engaged system) is an arrangement found in Romanesque architecture, where the internal space of a church is subdivided into three spaces, the nave and two side aisles, with aisles having half the width of the nave. The aisles are divided into bays, as is the nave, but each nave bay encompasses two aisle bays, with the arcade arches between the nave and aisles being grouped in pairs. This means that the bays of both the nave and aisles are square. This was necessary for churches roofed with groin vaults, as such vaults can only cover square bays, becoming badly distorted if they are raised over a rectangular space. The system was particularly used in Germany, where groin-vaulted naves were more common, as at Speyer Cathedral. The system largely fell out of use after the rib vault superseded the groin vault in the 12th century, as rib vaults with pointed arches can more easily cover rectangular bays.
Double-bay systems in Romanesque churches are almost always reflected in the alternation of supports, usually between compound piers and round columns, although some researchers see purely decorative alternation in some buildings, like the unvaulted Ely Cathedral.
