Double-bay system

In architecture, the double bay system (or engaged system) is the arrangement commonly found in Romanesque architecture, where the internal space of basilicas is subdivided into three spaces, the nave and two side aisles, with aisles having half the width of the nave. This arrangement required the ribbed vaults in the aisles to be twice smaller as well, so supports in the side aisles had to be spaced at half the step of the supports in the nave.^[1]

Double bay system inside the Speyer Cathedral includes alternation of supports

A partial plan of a church using the double bay system (Speyer Cathedral). The nave in the center uses large vaults (green), while side aisles use half-sized vaults (blue).The nave exhibits the alternation of supports (black), where the supports carrying the large vault are thicker than the ones only carrying the smaller vaults

Double-bay systems in Romanesque churches are almost always reflected in the alternation of supports, usually between compound piers and round columns,^[2] although some researchers see to purely decorative alternation in some buildings, like the Ely Cathedral.^[3]