(CE: 194a-226a) 
Triconch. A triconch is a square, oblong, or circular room expanded on three sides by semicircular exedrae, most frequently covered by a semidome. In the oldest examples, the central room is unroofed, as in the Well of Herodes Atticus in Corinth (second century A.D.). Later it usually had a wooden saddleback or barrel-vaulted roof. Very few central rooms were domed.
The triconch was first developed in the Roman Empire. Because of its impressive spatial effect, it was used principally in palace architecture for banquet and entertainment halls. The three exedrae were well suited to hold triclinia (three-part couches for dining). Such triconchs were usually situated on one side of a peristyle. Examples of such triconchs are known from the fourth to the seventh century. Examples in Egypt are the main halls of the two monastery complexes in Hilwan.
In some late-Roman palaces of the western part of the empire, the triconch is pushed back right to the edge of the edifice, where it presumably functioned as a private dining room. The hypothesis that only members of the imperial family were entitled to the use of a triconch (Lavin, 1962) has so far not been confirmed.
In Palestine, Asia Minor, and Egypt in the Roman period, the triconch was also used for ecclesiastical structures—for the sanctuary at the eastern end of the Christian basilica. According to the significance given to the liturgical side rooms, the triconch occupied the full breadth of the church or only the nave. The altar stood at its center. In Egypt—where such supplementary rooms were needed, one on each side of the central niche—the triconch was narrower than the breadth of the church. If it was much
narrower than the nave, an additional row of columns was set up in front of the sanctuary.
In principle, the triconch need not be composed of functionally homogeneous parts. Thus, in Tur ‘Abdin the naos, ordinarily shaped like a broad, shallow room and covered by a transverse, barrel-vaulted roof, was combined with the normally semicircular apse in the middle of the eastern wall into a triconch. A similar merging of heterogeneous rooms into a triconch is exhibited in the design of the sanctuary in the medieval Egyptian churches. Here the khurus was conjoined with the central niche into a triconch. Moreover, the ground plan in both cases is often rectangular. The conch shape, which in itself is based on the idea of a semicircular space, is then articulated only by the vault. Nevertheless, a triconch is not present if the broad room before the apse itself only forms part of a greater architectural form, as in the transept of the basilica of al-Ashmunayn and that at Hawwariyyah.
The triconch is also found in Byzantine churches. There, however, the individual conchs that form the rooms of the sanctuary are variously articulated. While the side conchas are for the most part shallow, the eastern concha is frequently stilted. There are also some examples from the high Middle Ages and later where the entire church is laid out as a triconch, as at Mount Athos. A similar situation obtains, moreover, in numerous churches in the West.
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