The apse. The apse was generally semicircular inside actually a semicircle completed by flat segments. In the case of simple rooms of small dimensions, it could be rectangular. We find, esp. in Egypt, some cases of trilobate apses. For some e.g., at Concordia Sagittaria in N Italy it was a case of extending, by adding aisles, an originally detached tribolate building which served as a martyrium. Beijing Map Tourist Attractions Finally we note some examples of internally polygonal apses e.g., S. Bertrand-de-Comminges in France. Externally the form differs considerably according to region.
In Italy and Greece the apse generally projects outward, but it can also be enclosed in a quadrangular block, externally semicircular or even polygonal, something which becomes frequent from the 5th c. in N Italy, on the Adriatic and all over the E Mediterranean and is typical of Constantinopolitan architecture. In some regions of Syria, N Africa and sometimes in Spain, a rectilinear “chevet” plan at ground level, not raised prevails, in which the apse is framed by two secondary rooms, connecting with it or not, which complete the rectangle. These rooms are often given the name “pastophories”: sometimes a distinction is made between the diakonikon service room for clergy and the prothesis premises for collection of offerings.
This terminology corresponds to an Eastern liturgy, confined in time and space, and cannot always be extended to this tripartite plan, whose justification is primarily architectural. Indeed, the side rooms did not have a fixed function: in N Syria the righthand room was frequently turned into a martyrium; in N Africa and Spain the baptistery was sometimes put there. We must distinguish this tripartite “chevet” single apse and side rooms from another tripartite type, later and particularly Eastern, with three projecting apses corresponding to three aisles. This type recurs in early medieval Western architecture. The height of the apse also varies according to region, depending on liturgical needs and architectural uses.