The medieval church had the apse area as its only liturgical space. In smaller religious buildings, the apse had two small lateral service rooms for the safekeeping of sacred books and preparing the liturgy. In larger churches and cathedrals, these areas were enlarged, as in the case of the Cefalù Cathedral, through the construction of two apses mirroring the central one. Thus, in liturgical practice, the Prothesis apse and the Diaconicon apse took shape. The first contained the liturgical objects intended for the offertory and the Eucharist, while the second was furnished with cabinets containing the vestments of the officiants and the sacred books.
