FENESTELLA CONFESSIONIS. - Both in block altars and in box altars there is sometimes a rectangular opening lined with marble, which made it possible to bring objects close to the tomb or relics. One of the earliest certain examples is provided by the box altar erected over the tomb of the martyrs in the basilica of S. Alessandro on the Nomentana.
Other examples occur in block altars, such as those in the cemetery of Panfilo and in S. Maria Antiqua and S. Maria in Via Lata in Rome, in the crypt of S. Aspreno in Naples, in the church of S. Pietro at Bagnacavallo, in the cathedral of Parenzo, etc., and in the basilica of the martyrs at Cimitile, etc. In semi-annular confessiones there are sometimes two f. c., one in the altar of the crypt, the other in the altar of the raised presbytery.
But the f. could also be that which allowed prayer directly upon the martyr’s tomb, as is learned from s. Gregorio di Tours concerning the tomb of S. Pietro in the Vatican: whoever wishes to pray, once the gates protecting it have been opened, enters above the tomb through such a «f. parvula patefacta, immisso intrersum cepite, quae necessitas prompt effagitat» (In gloria martyrum, 27: MGH, Scriptores verum Meroving, I, p. 504). The same author describes an analogous f. c. in the basilica of SS. Venerando e Nepotiano at Clermont (De gloria confessorum, 38: PL 71, 858). Sozomeno writes that in the upper part of the confessio of the Martyrs of Sebaste there was a foramen, that is, the equivalent of an f. c.