In Byzantine church architecture, the prothesis is the chamber beside the sanctuary used for preparing the Eucharistic bread and wine.
The prothesis is usually located on the north side of the central sanctuary, while the diakonikon occupies the corresponding south side. Together, these side chambers form the customary tripartite arrangement of the Byzantine sanctuary in many churches.
The preparation of the Eucharistic gifts takes place during the Proskomide, a liturgical rite associated with the prothesis. The chamber’s function is therefore both architectural and ceremonial.
The prothesis should not be confused with the bema, the sanctuary area in front of the altar, or the narthex, the entrance space at the west end of a church. Arrangements varied by period and region, especially in smaller churches.