The Prothesis is a sacred and ritual space located in the presbytery, specifically in the left apse, while the Diaconicon is located in the right apse. The Prothesis, Diaconicon and Bema constituted the three curved volumes of the presbytery, according to a solution widely used in early medieval churches. The combination of the Diaconicon and Prothesis is called pastophoria. The Prothesis is the place where the sacred objects, with which the liturgy of bread and wine is officiated and for the celebration of the Offertory, are kept. The viaticum, i.e. the shrine designed to house the Eucharist, but which could also contain the relics of a saint, was also kept in this room. Later, this part of the presbytery housed the Eucharist and became the Altar of the Sacrament.