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.

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 Ely Cathedral.