What architectural plan became standard for many middle Byzantine churches?

The cross-in-square plan became standard for many middle Byzantine churches.

The design places a domed central bay at the crossing of four barrel-vaulted arms, creating the shape of a cross within a nearly square building. Smaller domes or vaulted compartments often occupy the corner bays.

This plan developed from earlier Byzantine experiments with centralized and basilican buildings. It became especially common from the ninth century onward and was adapted across the Byzantine Empire and Orthodox Christian world.

The term does not mean that every church had identical dimensions. Some examples have elongated eastern or western bays, while later regional versions changed the proportions and decoration. The plan is also distinct from a simple basilica, whose main space is usually a long nave with parallel aisles.