PROTRO. — A protro is called a small annex attached to the entrance of a church building, a quadriporticus, or a portico; in its simplest form, it consists of a vault supported by two or more piers or columns.
The earliest examples of a protro are found in the Eastern Christian architecture of the first centuries. One dating to the 5th century is that of the church of Echmiadzin, consisting of a small tower attached to the façade of the building. Even the side of the quadriporticus of the ancient Vatican Basilica, in the ideal reconstructions of that structure, was to have had a protro. In any case, in Italy it appears already in the church of SS. Marini at Cimitile in the 8th century, and then spreads widely during the Romanesque period. This occurs not only in the characteristic and very simple examples of the Roman churches of S. Prassede, S. Maria in Cosmedin, S. Clemente, and S. Cosimato, where the protro truly had a protective function for those who lingered outside the church, or, when the protro had two orders, was the place where the guardian who watched over the church entrance stood; but also in those of northern Italy, where, as at Cremona, Piacenza, Modena, Bergamo, Ferrara, Verona, etc., it assumed the dignity of a decorative element intended to give grandeur to the entrance of the building. Among the most beautiful in this sense are that of S. Ciriaco in Ancona, a work of the 13th century, and that of the side door of the parish church of S. Quirico d’Orcia, whose caryatids supporting the small vault are attributed to the school of Giovanni Pisano. — See pl. IV.