ATRIO (a-trium, αἰδρίον). — In the Christian architecture of the early centuries, it consisted of an open space directly in front of the church façade and surrounded by porticoes that bounded its quadrangular area. The porticoes of the a. occupied the entire front of the church and were not especially visible, because they were enclosed by a continuous wall, interrupted only by the openings
for the doors. The roofing of the porticoes was supported by these perimeter walls and, on the inner side, by isolated columns arranged parallel to the enclosing walls and equally spaced.
The diffusion of the a. was characteristic, if not exclusive, of the Christian West: in Rome it characterized all the major basilicas. For this reason, too, the formal analogies between the a. of the early Christian church and that of the Roman house are evident. The a. was also customarily called parndisus (whence the French term pareis), perhaps because it was often laid out as a garden, or else, in Greek, αὐτή. narthex (v.) and extended along three sides (triporticum) or, more often, along all four (quadriporticum). A fountain in the middle, more commonly called a cantharus (v.), served for ablutions.
The ritual purpose of the a. is illustrated by the sacred writers, as a place initially intended to gather catechumens and penitents; its purpose therefore corresponded precisely to its architectural function as an intermediary element, intended to separate the sacred building from the public streets, thus distancing it from profane external life. In the East, however, this same purpose was more often achieved through the adoption of the peribolos (v.).
Around the seventh century, as the liturgical reasons for which it had been created ceased to apply, the a. began to diminish in importance and then to disappear; nevertheless, it still appears sporadically in pre-Romanesque and Romanesque churches, as in the well-known examples of the basilicas of S. Clemente in Rome, S. Ambrogio in Milan, and the cathedral of Salerno. A derivative of it may also be discerned in monastic cloisters which, presenting the same planimetric arrangement, were in all probability modeled on the a. of the early Christian church,
but deprived of its specific function. In the Renaissance as well (the churches of the S.ma Annunziata and S. Maria Maddalena dei Pazzi in Florence, and of S. Gregorio al Celio in Rome), the a. was sometimes placed in front of churches, with an ever-increasing variety of adaptations, so much so that in common usage the term also denotes a simple portico or any space preceding the nave of churches. — Vedi Tav. XXVII.